diff --git a/sqlglot/dialects/bigquery.py b/sqlglot/dialects/bigquery.py
index 99225ef85d..ba61cd1649 100644
--- a/sqlglot/dialects/bigquery.py
+++ b/sqlglot/dialects/bigquery.py
@@ -545,6 +545,9 @@ class Parser(parser.Parser):
             "EDIT_DISTANCE": _build_levenshtein,
             "FORMAT_DATE": _build_format_time(exp.TsOrDsToDate),
             "GENERATE_ARRAY": exp.GenerateSeries.from_arg_list,
+            "GREATEST": lambda args: exp.Greatest(
+                this=seq_get(args, 0), expressions=args[1:], return_null_if_any_null=True
+            ),
             "JSON_EXTRACT_SCALAR": _build_extract_json_with_default_path(exp.JSONExtractScalar),
             "JSON_EXTRACT_ARRAY": _build_extract_json_with_default_path(exp.JSONExtractArray),
             "JSON_EXTRACT_STRING_ARRAY": _build_extract_json_with_default_path(exp.JSONValueArray),
diff --git a/sqlglot/dialects/duckdb.py b/sqlglot/dialects/duckdb.py
index 2df0f33963..564c0517e8 100644
--- a/sqlglot/dialects/duckdb.py
+++ b/sqlglot/dialects/duckdb.py
@@ -141,6 +141,11 @@ def _build_make_timestamp(args: t.List) -> exp.Expression:
     )
 
 
+def _build_greatest(args: t.List) -> exp.Greatest:
+    """Build GREATEST with all arguments properly distributed."""
+    return exp.Greatest(this=seq_get(args, 0), expressions=args[1:])
+
+
 def _show_parser(*args: t.Any, **kwargs: t.Any) -> t.Callable[[DuckDB.Parser], exp.Show]:
     def _parse(self: DuckDB.Parser) -> exp.Show:
         return self._parse_show_duckdb(*args, **kwargs)
@@ -405,6 +410,29 @@ def _initcap_sql(self: DuckDB.Generator, expression: exp.Initcap) -> str:
     return _build_capitalization_sql(self, this_sql, escaped_delimiters_sql)
 
 
+def _greatest_sql(self: DuckDB.Generator, expression: exp.Greatest) -> str:
+    """
+    Handle GREATEST function with dialect-aware NULL behavior.
+
+    - If return_null_if_any_null=True (BigQuery-style): return NULL if any argument is NULL
+    - If return_null_if_any_null=False (DuckDB/PostgreSQL-style): ignore NULLs, return greatest non-NULL value
+    """
+    # Get all arguments
+    all_args = [expression.this] + (expression.expressions or [])
+    greatest_sql = self.func("GREATEST", *all_args)
+
+    if expression.args.get("return_null_if_any_null"):
+        # BigQuery behavior: NULL if any argument is NULL
+        case_expr = exp.case().when(
+            exp.or_(*[arg.is_(exp.null()) for arg in all_args], copy=False), exp.null(), copy=False
+        )
+        case_expr.set("default", greatest_sql)
+        return self.sql(case_expr)
+
+    # DuckDB/PostgreSQL behavior: use native GREATEST (ignores NULLs)
+    return self.sql(greatest_sql)
+
+
 class DuckDB(Dialect):
     NULL_ORDERING = "nulls_are_last"
     SUPPORTS_USER_DEFINED_TYPES = True
@@ -519,6 +547,7 @@ class Parser(parser.Parser):
 
         FUNCTIONS = {
             **parser.Parser.FUNCTIONS,
+            "GREATEST": _build_greatest,
             "ANY_VALUE": lambda args: exp.IgnoreNulls(this=exp.AnyValue.from_arg_list(args)),
             "ARRAY_REVERSE_SORT": _build_sort_array_desc,
             "ARRAY_SORT": exp.SortArray.from_arg_list,
@@ -866,6 +895,7 @@ class Generator(generator.Generator):
             exp.EuclideanDistance: rename_func("LIST_DISTANCE"),
             exp.GenerateDateArray: _generate_datetime_array_sql,
             exp.GenerateTimestampArray: _generate_datetime_array_sql,
+            exp.Greatest: _greatest_sql,
             exp.GroupConcat: lambda self, e: groupconcat_sql(self, e, within_group=False),
             exp.Explode: rename_func("UNNEST"),
             exp.IntDiv: lambda self, e: self.binary(e, "//"),
diff --git a/sqlglot/expressions.py b/sqlglot/expressions.py
index 92ee5fac9a..cf1dcfceac 100644
--- a/sqlglot/expressions.py
+++ b/sqlglot/expressions.py
@@ -6694,7 +6694,7 @@ class Getbit(Func):
 
 
 class Greatest(Func):
-    arg_types = {"this": True, "expressions": False}
+    arg_types = {"this": True, "expressions": False, "return_null_if_any_null": False}
     is_var_len_args = True
 
 
diff --git a/tests/dialects/test_bigquery.py b/tests/dialects/test_bigquery.py
index 3faccc5732..08229ddb02 100644
--- a/tests/dialects/test_bigquery.py
+++ b/tests/dialects/test_bigquery.py
@@ -2048,6 +2048,14 @@ def test_bigquery(self):
             },
         )
 
+        self.validate_all(
+            "SELECT GREATEST(1, NULL, 3)",
+            write={
+                "duckdb": "SELECT CASE WHEN 1 IS NULL OR NULL IS NULL OR 3 IS NULL THEN NULL ELSE GREATEST(1, NULL, 3) END",
+                "bigquery": "SELECT GREATEST(1, NULL, 3)",
+            },
+        )
+
     def test_errors(self):
         with self.assertRaises(ParseError):
             self.parse_one("SELECT * FROM a - b.c.d2")
diff --git a/tests/dialects/test_duckdb.py b/tests/dialects/test_duckdb.py
index 07e4fd18d8..d52d48c460 100644
--- a/tests/dialects/test_duckdb.py
+++ b/tests/dialects/test_duckdb.py
@@ -1238,6 +1238,7 @@ def test_duckdb(self):
         self.validate_identity(
             "SELECT CAST(TRIM(CAST(CAST('***apple***' AS BLOB) AS TEXT), CAST(CAST('*' AS BLOB) AS TEXT)) AS BLOB) AS result"
         )
+        self.validate_identity("SELECT GREATEST(1.0, 2.5, NULL, 3.7)")
 
     def test_array_index(self):
         with self.assertLogs(helper_logger) as cm: