diff --git a/.changelog/5513.fixed b/.changelog/5513.fixed
new file mode 100644
index 0000000000..6af842a7e8
--- /dev/null
+++ b/.changelog/5513.fixed
@@ -0,0 +1 @@
+`opentelemetry-sdk`: fix misleading instrument name validation error message (name max length is 255, not 63).
diff --git a/opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/instrument.py b/opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/instrument.py
index 48a56f62ff..c3ee9c47ae 100644
--- a/opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/instrument.py
+++ b/opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/instrument.py
@@ -46,7 +46,8 @@
 _logger = getLogger(__name__)
 
 
-_ERROR_MESSAGE = "Expected ASCII string of maximum length 63 characters but got {}"
+_NAME_ERROR_MESSAGE = "Expected ASCII string of maximum length 255 characters but got {}"
+_UNIT_ERROR_MESSAGE = "Expected ASCII string of maximum length 63 characters but got {}"
 
 
 @runtime_checkable
@@ -73,11 +74,11 @@ def __init__(
 
         if result["name"] is None:
             # pylint: disable=broad-exception-raised
-            raise Exception(_ERROR_MESSAGE.format(name))
+            raise Exception(_NAME_ERROR_MESSAGE.format(name))
 
         if result["unit"] is None:
             # pylint: disable=broad-exception-raised
-            raise Exception(_ERROR_MESSAGE.format(unit))
+            raise Exception(_UNIT_ERROR_MESSAGE.format(unit))
 
         name = result["name"]
         unit = result["unit"]
@@ -112,11 +113,11 @@ def __init__(
 
         if result["name"] is None:
             # pylint: disable=broad-exception-raised
-            raise Exception(_ERROR_MESSAGE.format(name))
+            raise Exception(_NAME_ERROR_MESSAGE.format(name))
 
         if result["unit"] is None:
             # pylint: disable=broad-exception-raised
-            raise Exception(_ERROR_MESSAGE.format(unit))
+            raise Exception(_UNIT_ERROR_MESSAGE.format(unit))
 
         name = result["name"]
         unit = result["unit"]
diff --git a/opentelemetry-sdk/tests/metrics/test_instrument.py b/opentelemetry-sdk/tests/metrics/test_instrument.py
index b6fd8dfe88..aa180a5d88 100644
--- a/opentelemetry-sdk/tests/metrics/test_instrument.py
+++ b/opentelemetry-sdk/tests/metrics/test_instrument.py
@@ -559,3 +559,17 @@ def test_disallow_direct_histogram_creation(self):
         with self.assertRaises(TypeError):
             # pylint: disable=abstract-class-instantiated
             Histogram("name", Mock(), Mock())
+
+
+class TestInstrumentValidationMessages(TestCase):
+    def test_invalid_name_error_message(self):
+        with self.assertRaises(Exception) as ctx:
+            _Counter("1-invalid-name", Mock(), Mock())
+        self.assertIn("maximum length 255", str(ctx.exception))
+        self.assertNotIn("63", str(ctx.exception))
+
+    def test_invalid_unit_error_message(self):
+        with self.assertRaises(Exception) as ctx:
+            _Counter("name", Mock(), Mock(), unit="u" * 64)
+        self.assertIn("maximum length 63", str(ctx.exception))
+        self.assertNotIn("255", str(ctx.exception))