diff --git a/src/pytest_mypy/__init__.py b/src/pytest_mypy/__init__.py
index 7b35ea8..0d661d1 100644
--- a/src/pytest_mypy/__init__.py
+++ b/src/pytest_mypy/__init__.py
@@ -16,10 +16,10 @@
     from typing import (
         Any,
         Dict,
+        IO,
         Iterator,
         List,
         Optional,
-        TextIO,
         Tuple,
         Union,
     )
@@ -297,6 +297,7 @@ class MypyResults:
     """Parsed results from Mypy."""
 
     _abspath_errors_type = typing.Dict[str, typing.List[str]]
+    _encoding = "utf-8"
 
     opts: List[str]
     stdout: str
@@ -305,14 +306,14 @@ class MypyResults:
     abspath_errors: _abspath_errors_type
     unmatched_stdout: str
 
-    def dump(self, results_f: TextIO) -> None:
+    def dump(self, results_f: IO[bytes]) -> None:
         """Cache results in a format that can be parsed by load()."""
-        return json.dump(vars(self), results_f)
+        results_f.write(json.dumps(vars(self)).encode(self._encoding))
 
     @classmethod
-    def load(cls, results_f: TextIO) -> MypyResults:
+    def load(cls, results_f: IO[bytes]) -> MypyResults:
         """Get results cached by dump()."""
-        return cls(**json.load(results_f))
+        return cls(**json.loads(results_f.read().decode(cls._encoding)))
 
     @classmethod
     def from_mypy(
@@ -360,7 +361,7 @@ def from_session(cls, session: pytest.Session) -> MypyResults:
         mypy_results_path = session.config.stash[stash_key["config"]].mypy_results_path
         with FileLock(str(mypy_results_path) + ".lock"):
             try:
-                with open(mypy_results_path, mode="r") as results_f:
+                with open(mypy_results_path, mode="rb") as results_f:
                     results = cls.load(results_f)
             except FileNotFoundError:
                 results = cls.from_mypy(
@@ -370,7 +371,7 @@ def from_session(cls, session: pytest.Session) -> MypyResults:
                         if isinstance(item, MypyFileItem)
                     ],
                 )
-                with open(mypy_results_path, mode="w") as results_f:
+                with open(mypy_results_path, mode="wb") as results_f:
                     results.dump(results_f)
         return results
 
@@ -393,7 +394,7 @@ def pytest_terminal_summary(
         """Report mypy results."""
         mypy_results_path = config.stash[stash_key["config"]].mypy_results_path
         try:
-            with open(mypy_results_path, mode="r") as results_f:
+            with open(mypy_results_path, mode="rb") as results_f:
                 results = MypyResults.load(results_f)
         except FileNotFoundError:
             # No MypyItems executed.
diff --git a/tests/test_pytest_mypy.py b/tests/test_pytest_mypy.py
index ed909c7..81c538d 100644
--- a/tests/test_pytest_mypy.py
+++ b/tests/test_pytest_mypy.py
@@ -10,6 +10,7 @@
 
 
 MYPY_VERSION = Version(mypy.version.__version__)
+PYTEST_VERSION = Version(pytest.__version__)
 PYTHON_VERSION = Version(
     ".".join(
         str(token)
@@ -60,6 +61,30 @@ def pyfunc(x: int) -> int:
     assert result.ret == pytest.ExitCode.OK
 
 
+@pytest.mark.skipif(
+    PYTEST_VERSION < Version("7.4"),
+    reason="https://github.com/pytest-dev/pytest/pull/10935",
+)
+@pytest.mark.skipif(
+    PYTHON_VERSION < Version("3.10"),
+    reason="PEP 597 was added in Python 3.10.",
+)
+@pytest.mark.skipif(
+    PYTHON_VERSION >= Version("3.12") and MYPY_VERSION < Version("1.5"),
+    reason="https://github.com/python/mypy/pull/15558",
+)
+def test_mypy_encoding_warnings(testdir, monkeypatch):
+    """Ensure no warnings are detected by PYTHONWARNDEFAULTENCODING."""
+    testdir.makepyfile("")
+    monkeypatch.setenv("PYTHONWARNDEFAULTENCODING", "1")
+    result = testdir.runpytest_subprocess("--mypy")
+    mypy_file_checks = 1
+    mypy_status_check = 1
+    mypy_checks = mypy_file_checks + mypy_status_check
+    expected_warnings = 2  # https://github.com/python/mypy/issues/14603
+    result.assert_outcomes(passed=mypy_checks, warnings=expected_warnings)
+
+
 def test_mypy_pyi(testdir, xdist_args):
     """
     Verify that a .py file will be skipped if
@@ -524,7 +549,7 @@ def test_mypy_no_output(testdir, xdist_args):
             def pytest_configure(config):
                 pytest_mypy = config.pluginmanager.getplugin("mypy")
                 mypy_config_stash = config.stash[pytest_mypy.stash_key["config"]]
-                with open(mypy_config_stash.mypy_results_path, mode="w") as results_f:
+                with open(mypy_config_stash.mypy_results_path, mode="wb") as results_f:
                     pytest_mypy.MypyResults(
                         opts=[],
                         stdout="",
@@ -602,7 +627,7 @@ def test_mypy_xfail_reports_stdout(testdir, xdist_args):
             def pytest_configure(config):
                 pytest_mypy = config.pluginmanager.getplugin("mypy")
                 mypy_config_stash = config.stash[pytest_mypy.stash_key["config"]]
-                with open(mypy_config_stash.mypy_results_path, mode="w") as results_f:
+                with open(mypy_config_stash.mypy_results_path, mode="wb") as results_f:
                     pytest_mypy.MypyResults(
                         opts=[],
                         stdout="{stdout}",