diff --git a/mypy.ini b/mypy.ini
index 05d59c9ea7b0..a8fa4f4ccfb9 100644
--- a/mypy.ini
+++ b/mypy.ini
@@ -95,14 +95,18 @@ ignore_missing_imports = True
 ignore_missing_imports = True
 
 [mypy-google.cloud.bigtable.*]
+# Broadly ignore errors for the older, legacy/handwritten parts of the library
 ignore_errors = True
 
 [mypy-google.cloud.bigtable.data.*]
+# Enable strict checks ONLY for the modern data submodule
 check_untyped_defs = True
 warn_unreachable = True
 disallow_any_generics = True
+# Do NOT ignore errors in the modern submodule
 ignore_errors = False
 
+
 [mypy-google.cloud.bigtable_admin.*]
 ignore_errors = True
 
diff --git a/packages/google-cloud-spanner/mypy.ini b/packages/google-cloud-spanner/mypy.ini
deleted file mode 100644
index e0e0da2e9e40..000000000000
--- a/packages/google-cloud-spanner/mypy.ini
+++ /dev/null
@@ -1,15 +0,0 @@
-[mypy]
-python_version = 3.14
-namespace_packages = True
-ignore_missing_imports = False
-
-# TODO(https://github.com/googleapis/gapic-generator-python/issues/2563):
-# Dependencies that historically lacks py.typed markers
-[mypy-google.iam.*]
-ignore_missing_imports = True
-
-# Helps mypy navigate the 'google' namespace more reliably in 3.10+
-explicit_package_bases = True
-
-# Performance: reuse results from previous runs to speed up 'nox'
-incremental = True
diff --git a/packages/google-cloud-spanner/noxfile.py b/packages/google-cloud-spanner/noxfile.py
index fa74716b8142..7dcb31b45ed7 100644
--- a/packages/google-cloud-spanner/noxfile.py
+++ b/packages/google-cloud-spanner/noxfile.py
@@ -69,6 +69,17 @@
 SYSTEM_TEST_EXTRAS_BY_PYTHON: Dict[str, List[str]] = {}
 
 CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
+# Path to the centralized mypy configuration file at the repository root.
+# Search upwards to support running nox from both monorepo packages and integration test goldens.
+MYPY_CONFIG_FILE = next(
+    (
+        str(p / "mypy.ini")
+        for p in CURRENT_DIRECTORY.parents
+        if (p / "mypy.ini").exists()
+    ),
+    str(CURRENT_DIRECTORY.parent.parent / "mypy.ini"),
+)
+
 
 nox.options.sessions = [
     "unit-3.10",
@@ -767,6 +778,7 @@ def mypy(session):
     session.install(".")
     session.run(
         "mypy",
+        f"--config-file={MYPY_CONFIG_FILE}",
         "-p",
         "google",
         # "--check-untyped-defs",
diff --git a/packages/google-cloud-spanner/tests/system/_helpers.py b/packages/google-cloud-spanner/tests/system/_helpers.py
index 4cd9879e5d2d..49c8c616709c 100644
--- a/packages/google-cloud-spanner/tests/system/_helpers.py
+++ b/packages/google-cloud-spanner/tests/system/_helpers.py
@@ -18,9 +18,9 @@
 import uuid
 
 from google.api_core import exceptions
+from google.cloud.spanner_v1 import instance as instance_mod
 from test_utils import retry
 
-from google.cloud.spanner_v1 import instance as instance_mod
 from tests import _fixtures
 
 CREATE_INSTANCE_ENVVAR = "GOOGLE_CLOUD_TESTS_CREATE_SPANNER_INSTANCE"
diff --git a/packages/google-cloud-spanner/tests/system/conftest.py b/packages/google-cloud-spanner/tests/system/conftest.py
index 740cfc0e02b5..f099f3977b58 100644
--- a/packages/google-cloud-spanner/tests/system/conftest.py
+++ b/packages/google-cloud-spanner/tests/system/conftest.py
@@ -17,7 +17,6 @@
 import time
 
 import pytest
-
 from google.cloud import spanner_v1
 from google.cloud.spanner_admin_database_v1 import DatabaseDialect
 from google.cloud.spanner_admin_database_v1.types.backup import (