diff --git a/docs/changelog/122425.yaml b/docs/changelog/122425.yaml
new file mode 100644
index 0000000000000..a0e590dcdc36c
--- /dev/null
+++ b/docs/changelog/122425.yaml
@@ -0,0 +1,5 @@
+pr: 122425
+summary: Fix synthetic source bug that would mishandle nested `dense_vector` fields
+area: Mapping
+type: bug
+issues: []
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/20_synthetic_source.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/20_synthetic_source.yml
index 90c0bc87c8ea3..97e6e96dc16f2 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/20_synthetic_source.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/indices.create/20_synthetic_source.yml
@@ -2118,3 +2118,143 @@ create index with use_synthetic_source:
         flush: false
   - gt: { test.store_size_in_bytes: 0 }
   - is_false: test.fields._recovery_source
+---
+"Nested synthetic source with indexed dense vectors":
+  - requires:
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: PUT
+          path: /{index}
+          capabilities: [ synthetic_nested_dense_vector_bug_fix ]
+      reason: "Requires synthetic source bugfix for dense vectors in nested objects"
+  - do:
+      indices.create:
+        index: nested_dense_vector_synthetic_test
+        body:
+          mappings:
+            properties:
+              parent:
+                type: nested
+                properties:
+                  vector:
+                    type: dense_vector
+                    index: true
+                    similarity: l2_norm
+                  text:
+                    type: text
+          settings:
+            index:
+              mapping:
+                source:
+                  mode: synthetic
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 0
+        refresh: true
+        body: { "parent": [ { "vector": [ 1, 2 ],"text": "foo" }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 1
+        refresh: true
+        body: { "parent": [ { "text": "foo" }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 2
+        refresh: true
+        body: { "parent": [ { "vector": [ 1, 2 ] }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+
+  - do:
+      search:
+        index: nested_dense_vector_synthetic_test
+        body:
+          query:
+            match_all: {}
+
+  - match: { hits.hits.0._source.parent.0.vector: [ 1.0, 2.0 ] }
+  - match: { hits.hits.0._source.parent.0.text: "foo" }
+  - match: { hits.hits.0._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.0._source.parent.1.text: "bar" }
+  - is_false:  hits.hits.1._source.parent.0.vector
+  - match: { hits.hits.1._source.parent.0.text: "foo" }
+  - match: { hits.hits.1._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.1._source.parent.1.text: "bar" }
+  - match: {hits.hits.2._source.parent.0.vector: [ 1.0, 2.0 ] }
+  - is_false: hits.hits.2._source.parent.0.text
+  - match: { hits.hits.2._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.2._source.parent.1.text: "bar" }
+---
+"Nested synthetic source with un-indexed dense vectors":
+  - requires:
+      test_runner_features: [ capabilities ]
+      capabilities:
+        - method: PUT
+          path: /{index}
+          capabilities: [ synthetic_nested_dense_vector_bug_fix ]
+      reason: "Requires synthetic source bugfix for dense vectors in nested objects"
+  - do:
+      indices.create:
+        index: nested_dense_vector_synthetic_test
+        body:
+          mappings:
+            properties:
+              parent:
+                type: nested
+                properties:
+                  vector:
+                    type: dense_vector
+                    index: false
+                  text:
+                    type: text
+          settings:
+            index:
+              mapping:
+                source:
+                  mode: synthetic
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 0
+        refresh: true
+        body: { "parent": [ { "vector": [ 1, 2 ],"text": "foo" }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 1
+        refresh: true
+        body: { "parent": [ { "text": "foo" }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+  - do:
+      index:
+        index: nested_dense_vector_synthetic_test
+        id: 2
+        refresh: true
+        body: { "parent": [ { "vector": [ 1, 2 ] }, { "vector": [ 2, 2 ], "text": "bar" } ] }
+
+
+  - do:
+      search:
+        index: nested_dense_vector_synthetic_test
+        body:
+          query:
+            match_all: {}
+
+  - match: { hits.hits.0._source.parent.0.vector: [ 1.0, 2.0 ] }
+  - match: { hits.hits.0._source.parent.0.text: "foo" }
+  - match: { hits.hits.0._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.0._source.parent.1.text: "bar" }
+  - is_false:  hits.hits.1._source.parent.0.vector
+  - match: { hits.hits.1._source.parent.0.text: "foo" }
+  - match: { hits.hits.1._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.1._source.parent.1.text: "bar" }
+  - match: {hits.hits.2._source.parent.0.vector: [ 1.0, 2.0 ] }
+  - is_false: hits.hits.2._source.parent.0.text
+  - match: { hits.hits.2._source.parent.1.vector: [ 2.0, 2.0 ] }
+  - match: { hits.hits.2._source.parent.1.text: "bar" }
+
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
index 6d01ea21a6478..ae6140a1cafe5 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java
@@ -2405,6 +2405,12 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
                     magnitudeReader = leafReader.getNumericDocValues(fullPath() + COSINE_MAGNITUDE_FIELD_SUFFIX);
                 }
                 return docId -> {
+                    if (values.docID() > docId) {
+                        return hasValue = false;
+                    }
+                    if (values.docID() == docId) {
+                        return hasValue = true;
+                    }
                     hasValue = docId == values.advance(docId);
                     hasMagnitude = hasValue && magnitudeReader != null && magnitudeReader.advanceExact(docId);
                     return hasValue;
@@ -2413,6 +2419,12 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
             byteVectorValues = leafReader.getByteVectorValues(fullPath());
             if (byteVectorValues != null) {
                 return docId -> {
+                    if (byteVectorValues.docID() > docId) {
+                        return hasValue = false;
+                    }
+                    if (byteVectorValues.docID() == docId) {
+                        return hasValue = true;
+                    }
                     hasValue = docId == byteVectorValues.advance(docId);
                     return hasValue;
                 };
@@ -2474,6 +2486,12 @@ public DocValuesLoader docValuesLoader(LeafReader leafReader, int[] docIdsInLeaf
                 return null;
             }
             return docId -> {
+                if (values.docID() > docId) {
+                    return hasValue = false;
+                }
+                if (values.docID() == docId) {
+                    return hasValue = true;
+                }
                 hasValue = docId == values.advance(docId);
                 return hasValue;
             };
diff --git a/server/src/main/java/org/elasticsearch/rest/action/admin/indices/CreateIndexCapabilities.java b/server/src/main/java/org/elasticsearch/rest/action/admin/indices/CreateIndexCapabilities.java
index 900a352d42f30..334e68648d853 100644
--- a/server/src/main/java/org/elasticsearch/rest/action/admin/indices/CreateIndexCapabilities.java
+++ b/server/src/main/java/org/elasticsearch/rest/action/admin/indices/CreateIndexCapabilities.java
@@ -26,5 +26,11 @@ public class CreateIndexCapabilities {
      */
     private static final String LOOKUP_INDEX_MODE_CAPABILITY = "lookup_index_mode";
 
-    public static Set<String> CAPABILITIES = Set.of(LOGSDB_INDEX_MODE_CAPABILITY, LOOKUP_INDEX_MODE_CAPABILITY);
+    private static final String NESTED_DENSE_VECTOR_SYNTHETIC_TEST = "nested_dense_vector_synthetic_test";
+
+    public static final Set<String> CAPABILITIES = Set.of(
+        LOGSDB_INDEX_MODE_CAPABILITY,
+        LOOKUP_INDEX_MODE_CAPABILITY,
+        NESTED_DENSE_VECTOR_SYNTHETIC_TEST
+    );
 }