diff --git a/moto/dynamodb/models/__init__.py b/moto/dynamodb/models/__init__.py
index de9c51cea..a5f49b66e 100644
--- a/moto/dynamodb/models/__init__.py
+++ b/moto/dynamodb/models/__init__.py
@@ -1485,13 +1485,6 @@ class DynamoDBBackend(BaseBackend):
             filter_expression, expr_names, expr_values
         )
 
-        projection_expression = ",".join(
-            [
-                expr_names.get(attr, attr)
-                for attr in projection_expression.replace(" ", "").split(",")
-            ]
-        )
-
         return table.scan(
             scan_filters,
             limit,
diff --git a/moto/dynamodb/responses.py b/moto/dynamodb/responses.py
index 38435a99b..93fa9b51a 100644
--- a/moto/dynamodb/responses.py
+++ b/moto/dynamodb/responses.py
@@ -7,6 +7,7 @@ from functools import wraps
 
 from moto.core.responses import BaseResponse
 from moto.core.utils import camelcase_to_underscores, amz_crc32, amzn_request_id
+from moto.dynamodb.parsing.reserved_keywords import ReservedKeywords
 from .exceptions import (
     MockValidationException,
     ResourceNotFoundException,
@@ -100,6 +101,21 @@ def put_has_empty_attrs(field_updates, table):
     return False
 
 
+def check_projection_expression(expression):
+    if expression.upper() in ReservedKeywords.get_reserved_keywords():
+        raise MockValidationException(
+            f"ProjectionExpression: Attribute name is a reserved keyword; reserved keyword: {expression}"
+        )
+    if expression[0].isnumeric():
+        raise MockValidationException(
+            "ProjectionExpression: Attribute name starts with a number"
+        )
+    if " " in expression:
+        raise MockValidationException(
+            "ProjectionExpression: Attribute name contains white space"
+        )
+
+
 class DynamoHandler(BaseResponse):
     def get_endpoint_name(self, headers):
         """Parses request headers and extracts part od the X-Amz-Target
@@ -728,14 +744,17 @@ class DynamoHandler(BaseResponse):
                 else expression
             )
 
-        if projection_expression and expr_attr_names:
+        if projection_expression:
             expressions = [x.strip() for x in projection_expression.split(",")]
-            return ",".join(
-                [
-                    ".".join([_adjust(expr) for expr in nested_expr.split(".")])
-                    for nested_expr in expressions
-                ]
-            )
+            for expression in expressions:
+                check_projection_expression(expression)
+            if expr_attr_names:
+                return ",".join(
+                    [
+                        ".".join([_adjust(expr) for expr in nested_expr.split(".")])
+                        for nested_expr in expressions
+                    ]
+                )
 
         return projection_expression
 
@@ -760,6 +779,10 @@ class DynamoHandler(BaseResponse):
         limit = self.body.get("Limit")
         index_name = self.body.get("IndexName")
 
+        projection_expression = self._adjust_projection_expression(
+            projection_expression, expression_attribute_names
+        )
+
         try:
             items, scanned_count, last_evaluated_key = self.dynamodb_backend.scan(
                 name,
diff --git a/tests/test_dynamodb/test_dynamodb.py b/tests/test_dynamodb/test_dynamodb.py
index 75e5515cc..d359e49f6 100644
--- a/tests/test_dynamodb/test_dynamodb.py
+++ b/tests/test_dynamodb/test_dynamodb.py
@@ -5780,3 +5780,63 @@ def test_projection_expression_execution_order():
         ProjectionExpression="#a",
         ExpressionAttributeNames={"#a": "hashKey"},
     )
+
+
+@mock_dynamodb
+def test_invalid_projection_expressions():
+    table_name = "test-projection-expressions-table"
+    client = boto3.client("dynamodb", region_name="us-east-1")
+    client.create_table(
+        TableName=table_name,
+        KeySchema=[{"AttributeName": "customer", "KeyType": "HASH"}],
+        AttributeDefinitions=[{"AttributeName": "customer", "AttributeType": "S"}],
+        ProvisionedThroughput={"ReadCapacityUnits": 5, "WriteCapacityUnits": 5},
+    )
+
+    with pytest.raises(
+        ClientError,
+        match="ProjectionExpression: Attribute name is a reserved keyword; reserved keyword: name",
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="name")
+    with pytest.raises(
+        ClientError, match="ProjectionExpression: Attribute name starts with a number"
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="3ame")
+    with pytest.raises(
+        ClientError, match="ProjectionExpression: Attribute name contains white space"
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="na me")
+
+    with pytest.raises(
+        ClientError,
+        match="ProjectionExpression: Attribute name is a reserved keyword; reserved keyword: name",
+    ):
+        client.get_item(
+            TableName=table_name,
+            Key={"customer": {"S": "a"}},
+            ProjectionExpression="name",
+        )
+
+    with pytest.raises(
+        ClientError,
+        match="ProjectionExpression: Attribute name is a reserved keyword; reserved keyword: name",
+    ):
+        client.query(
+            TableName=table_name,
+            KeyConditionExpression="a",
+            ProjectionExpression="name",
+        )
+
+    with pytest.raises(
+        ClientError,
+        match="ProjectionExpression: Attribute name is a reserved keyword; reserved keyword: name",
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="not_a_keyword, name")
+    with pytest.raises(
+        ClientError, match="ProjectionExpression: Attribute name starts with a number"
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="not_a_keyword, 3ame")
+    with pytest.raises(
+        ClientError, match="ProjectionExpression: Attribute name contains white space"
+    ):
+        client.scan(TableName=table_name, ProjectionExpression="not_a_keyword, na me")