diff --git a/pom.xml b/pom.xml
index 1587d56..441dc78 100644
--- a/pom.xml
+++ b/pom.xml
@@ -27,6 +27,7 @@
         <junit.version>5.13.2</junit.version>
         <slf4j-simple.version>2.0.17</slf4j-simple.version>
         <apache-commons-collections4.version>4.5.0</apache-commons-collections4.version>
+        <externalsortinginjava.version>0.6.2</externalsortinginjava.version>
         <maven-compiler-plugin.version>3.14.0</maven-compiler-plugin.version>
         <maven-dependency-plugin.version>3.8.1</maven-dependency-plugin.version>
         <maven-source-plugin.version>3.3.1</maven-source-plugin.version>
@@ -231,6 +232,13 @@
             <version>${tableschema-java-version}</version>
         </dependency>
 
+        <!-- Sorting -->
+        <dependency>
+            <groupId>com.google.code.externalsortinginjava</groupId>
+            <artifactId>externalsortinginjava</artifactId>
+            <version>${externalsortinginjava.version}</version>
+        </dependency>
+
         <!-- Unit Testing -->
         <dependency>
             <groupId>org.junit.jupiter</groupId>
diff --git a/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java b/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
index dd53e73..4d0e51c 100644
--- a/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
+++ b/src/main/java/io/frictionlessdata/datapackage/resource/AbstractResource.java
@@ -6,6 +6,7 @@
 import com.fasterxml.jackson.annotation.JsonProperty;
 import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.ObjectMapper;
+import com.google.code.externalsorting.ExternalSort;
 import io.frictionlessdata.datapackage.Dialect;
 import io.frictionlessdata.datapackage.JSONBase;
 import io.frictionlessdata.datapackage.Package;
@@ -14,10 +15,7 @@
 import io.frictionlessdata.datapackage.exceptions.DataPackageValidationException;
 import io.frictionlessdata.datapackage.fk.PackageForeignKey;
 import io.frictionlessdata.tableschema.Table;
-import io.frictionlessdata.tableschema.exception.ForeignKeyException;
-import io.frictionlessdata.tableschema.exception.JsonSerializingException;
-import io.frictionlessdata.tableschema.exception.TableIOException;
-import io.frictionlessdata.tableschema.exception.TypeInferringException;
+import io.frictionlessdata.tableschema.exception.*;
 import io.frictionlessdata.tableschema.field.Field;
 import io.frictionlessdata.tableschema.fk.ForeignKey;
 import io.frictionlessdata.tableschema.io.FileReference;
@@ -40,6 +38,7 @@
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.*;
+import java.util.stream.Collectors;
 
 /**
  * Abstract base implementation of a Resource.
@@ -368,6 +367,7 @@ public List<Table> getTables() throws Exception {
         return tables;
     }
 
+    @Override
     public void checkRelations(Package pkg) {
         if (null != schema) {
             List<PackageForeignKey> fks = new ArrayList<>();
@@ -439,6 +439,71 @@ public void checkRelations(Package pkg) {
         }
     }
 
+    @Override
+    public void checkPrimaryKeys() {
+        if (null != schema) {
+            Object pkObj = schema.getPrimaryKey();
+            if (pkObj == null) {
+                return; // no primary key defined
+            }
+
+            // Normalize PK fields
+            String[] pkFields;
+            if (pkObj instanceof String) {
+                pkFields = new String[]{(String) pkObj};
+            } else if (pkObj instanceof String[]) {
+                pkFields = (String[]) pkObj;
+            } else {
+                throw new PrimaryKeyException("Unsupported primary key type: " + pkObj.getClass());
+            }
+
+            try {
+                // Dump all keys to a temporary file
+                Path tempFile = Files.createTempFile("pk-check", ".txt");
+                try (BufferedWriter writer = Files.newBufferedWriter(tempFile)) {
+                    List<Object> data = this.getData(true, false, true, false);
+                    for (Object d : data) {
+                        Map<String, Object> row = (Map<String, Object>) d;
+                        String key = Arrays.stream(pkFields)
+                                .map(f -> String.valueOf(row.get(f)))
+                                .collect(Collectors.joining("\t"));
+                        writer.write(key);
+                        writer.newLine();
+                    }
+                }
+
+                // Use ExternalSort to sort the file
+                File inputFile = tempFile.toFile();
+                File sortedFile = Files.createTempFile("pk-check-sorted", ".txt").toFile();
+
+                List<File> tempChunks = ExternalSort.sortInBatch(inputFile);
+                ExternalSort.mergeSortedFiles(tempChunks, sortedFile);
+
+                // Scan sorted file line-by-line for duplicates
+                try (BufferedReader reader = new BufferedReader(new FileReader(sortedFile, StandardCharsets.UTF_8))) {
+                    String prev = null;
+                    String line;
+                    while ((line = reader.readLine()) != null) {
+                        if (line.equals(prev)) {
+                            throw new PrimaryKeyException(
+                                    "Primary key violation in resource '" + this.getName() +
+                                            "': duplicate key " + line
+                            );
+                        }
+                        prev = line;
+                    }
+                }
+
+                // Cleanup
+                Files.deleteIfExists(tempFile);
+                Files.deleteIfExists(sortedFile.toPath());
+
+            } catch (Exception e) {
+                throw new PrimaryKeyException("Error validating primary keys: " + e.getMessage());
+            }
+        }
+    }
+
     public void validate(Package pkg)  {
 
         try {
diff --git a/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java b/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
index fe5d73f..0544c73 100644
--- a/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
+++ b/src/main/java/io/frictionlessdata/datapackage/resource/Resource.java
@@ -364,6 +364,8 @@ static ResourceBuilder builder(String resourceName) {
 
     void checkRelations(Package pkg) throws Exception;
 
+    void checkPrimaryKeys() throws Exception;
+
     /**
      * Recreate a Resource object from a JSON descriptor, a base path to resolve relative file paths against
      * and a flag that tells us whether we are reading from inside a ZIP archive.
diff --git a/src/test/java/io/frictionlessdata/datapackage/PrimaryKeysTest.java b/src/test/java/io/frictionlessdata/datapackage/PrimaryKeysTest.java
new file mode 100644
index 0000000..4e02d61
--- /dev/null
+++ b/src/test/java/io/frictionlessdata/datapackage/PrimaryKeysTest.java
@@ -0,0 +1,71 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.frictionlessdata.datapackage;
+
+import io.frictionlessdata.datapackage.resource.Resource;
+import io.frictionlessdata.tableschema.exception.PrimaryKeyException;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
+
+import java.nio.file.Path;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class PrimaryKeysTest {
+
+    @Test
+    @DisplayName("Test the uniqueness of simple primary keys - invalid case")
+    void testPrimaryKeysUniqueInvalid() throws Exception {
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/primary-keys/simple/primary_keys_csv_invalid.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+
+        Throwable ex = assertThrows(Exception.class, teams::checkPrimaryKeys);
+        assertInstanceOf(PrimaryKeyException.class, ex);
+        assertEquals("Error validating primary keys: Primary key violation in resource 'teams': duplicate key 1", ex.getMessage());
+    }
+
+    @Test
+    @DisplayName("Test the uniqueness of simple primary keys - valid case")
+    void testPrimaryKeysUniqueValid() throws Exception {
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/primary-keys/simple/primary_keys_csv_valid.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+
+        assertDoesNotThrow(teams::checkPrimaryKeys);
+    }
+
+    @Test
+    @DisplayName("Test the uniqueness of composite primary keys - invalid case")
+    void testCompositePrimaryKeysUniqueInvalid() throws Exception {
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/primary-keys/composite/primary_keys_csv_invalid.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+
+        Throwable ex = assertThrows(Exception.class, teams::checkPrimaryKeys);
+        assertInstanceOf(PrimaryKeyException.class, ex);
+        assertEquals("Error validating primary keys: Primary key violation in resource 'teams': duplicate key UK\tLondon", ex.getMessage());
+    }
+
+    @Test
+    @DisplayName("Test the uniqueness of composite primary keys - valid case")
+    void testCompositePrimaryKeysUniqueValid() throws Exception {
+        Path resourcePath = TestUtil.getResourcePath("/fixtures/datapackages/primary-keys/composite/primary_keys_csv_valid.json");
+        Package pkg = new Package(resourcePath, true);
+        Resource teams = pkg.getResource("teams");
+
+        assertDoesNotThrow(teams::checkPrimaryKeys);
+    }
+}
diff --git a/src/test/java/io/frictionlessdata/datapackage/resource/NonTabularResourceTest.java b/src/test/java/io/frictionlessdata/datapackage/resource/NonTabularResourceTest.java
index 6f01c6d..384a186 100644
--- a/src/test/java/io/frictionlessdata/datapackage/resource/NonTabularResourceTest.java
+++ b/src/test/java/io/frictionlessdata/datapackage/resource/NonTabularResourceTest.java
@@ -736,6 +736,10 @@ public String getSerializationFormat() {
         public void checkRelations(Package aPackage) throws Exception {
         }
 
+        @Override
+        public void checkPrimaryKeys() throws Exception {
+        }
+
         @Override
         public void validate(Package aPackage) {
         }
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_invalid.json b/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_invalid.json
new file mode 100644
index 0000000..4264323
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_invalid.json
@@ -0,0 +1,29 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "profile": "tabular-data-resource",
+      "encoding": "UTF-8",
+      "format": "csv",
+      "schema": {
+        "fields": [
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "country",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "primaryKey": ["country", "city"]
+      },
+      "path": "teams.csv"
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_valid.json b/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_valid.json
new file mode 100644
index 0000000..07c1241
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/composite/primary_keys_csv_valid.json
@@ -0,0 +1,29 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "profile": "tabular-data-resource",
+      "encoding": "UTF-8",
+      "format": "csv",
+      "schema": {
+        "fields": [
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "country",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "primaryKey": ["country", "city"]
+      },
+      "path": "teams-valid.csv"
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/composite/teams-valid.csv b/src/test/resources/fixtures/datapackages/primary-keys/composite/teams-valid.csv
new file mode 100644
index 0000000..8a8159f
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/composite/teams-valid.csv
@@ -0,0 +1,4 @@
+name,country,city
+Arsenal,UK,London
+Real,Spain,Madrid
+Bayern,Germany,Munich
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/composite/teams.csv b/src/test/resources/fixtures/datapackages/primary-keys/composite/teams.csv
new file mode 100644
index 0000000..233f276
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/composite/teams.csv
@@ -0,0 +1,5 @@
+name,country,city
+Arsenal,UK,London
+Real,Spain,Madrid
+Bayern,Germany,Munich
+Chelsea,UK,London
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_invalid.json b/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_invalid.json
new file mode 100644
index 0000000..d51b947
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_invalid.json
@@ -0,0 +1,33 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "profile": "tabular-data-resource",
+      "encoding": "UTF-8",
+      "format": "csv",
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer",
+            "constraints": {
+              "required": true,
+              "unique": true
+            }
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "primaryKey": "id"
+      },
+      "path": "teams.csv"
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_valid.json b/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_valid.json
new file mode 100644
index 0000000..b2ad306
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/simple/primary_keys_csv_valid.json
@@ -0,0 +1,33 @@
+{
+  "name": "foreign-keys",
+  "resources": [
+    {
+      "name": "teams",
+      "profile": "tabular-data-resource",
+      "encoding": "UTF-8",
+      "format": "csv",
+      "schema": {
+        "fields": [
+          {
+            "name": "id",
+            "type": "integer",
+            "constraints": {
+              "required": true,
+              "unique": true
+            }
+          },
+          {
+            "name": "name",
+            "type": "string"
+          },
+          {
+            "name": "city",
+            "type": "string"
+          }
+        ],
+        "primaryKey": "id"
+      },
+      "path": "teams-valid.csv"
+    }
+  ]
+}
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/simple/teams-valid.csv b/src/test/resources/fixtures/datapackages/primary-keys/simple/teams-valid.csv
new file mode 100644
index 0000000..1d69a7d
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/simple/teams-valid.csv
@@ -0,0 +1,4 @@
+id,name,city
+1,Arsenal,London
+2,Real,Madrid
+3,Bayern,Munich
\ No newline at end of file
diff --git a/src/test/resources/fixtures/datapackages/primary-keys/simple/teams.csv b/src/test/resources/fixtures/datapackages/primary-keys/simple/teams.csv
new file mode 100644
index 0000000..07ea407
--- /dev/null
+++ b/src/test/resources/fixtures/datapackages/primary-keys/simple/teams.csv
@@ -0,0 +1,4 @@
+id,name,city
+1,Arsenal,London
+1,Real,Madrid
+1,Bayern,Munich
\ No newline at end of file