diff --git a/newsfragments/3555.bugfix.rst b/newsfragments/3555.bugfix.rst
new file mode 100644
index 0000000000..979ede1d5d
--- /dev/null
+++ b/newsfragments/3555.bugfix.rst
@@ -0,0 +1,2 @@
+`trio.Process` no longer emits an `EncodingWarning` on Linux when Python runs with
+``-X warn_default_encoding`` (or ``PYTHONWARNDEFAULTENCODING=1``).
diff --git a/src/trio/_subprocess.py b/src/trio/_subprocess.py
index d73ba3dc23..710600dcf4 100644
--- a/src/trio/_subprocess.py
+++ b/src/trio/_subprocess.py
@@ -7,6 +7,7 @@
 import warnings
 from contextlib import ExitStack
 from functools import partial
+from io import FileIO
 from typing import (
     TYPE_CHECKING,
     Final,
@@ -32,7 +33,6 @@
 if TYPE_CHECKING:
     import signal
     from collections.abc import Awaitable, Callable, Iterable, Mapping, Sequence
-    from io import TextIOWrapper
 
     from typing_extensions import Unpack
 
@@ -173,7 +173,7 @@ def __init__(
 
         self._wait_lock: Lock = Lock()
 
-        self._pidfd: TextIOWrapper | None = None
+        self._pidfd: FileIO | None = None
         if can_try_pidfd_open:
             try:
                 fd: int = pidfd_open(self._proc.pid, 0)
@@ -186,8 +186,7 @@ def __init__(
             else:
                 # It worked! Wrap the raw fd up in a Python file object to
                 # make sure it'll get closed.
-                # SIM115: open-file-with-context-handler
-                self._pidfd = open(fd)  # noqa: SIM115
+                self._pidfd = FileIO(fd)
 
         self.args: StrOrBytesPath | Sequence[StrOrBytesPath] = self._proc.args
         self.pid: int = self._proc.pid
diff --git a/src/trio/_tests/test_subprocess.py b/src/trio/_tests/test_subprocess.py
index dbd55f8282..be762c0756 100644
--- a/src/trio/_tests/test_subprocess.py
+++ b/src/trio/_tests/test_subprocess.py
@@ -157,6 +157,26 @@ async def test_basic_no_pidfd(background_process: BackgroundProcessType) -> None
         )
 
 
+async def test_no_encoding_warning() -> None:
+    # Spawning a process must not open anything in text mode without an
+    # explicit encoding, which warns under -X warn_default_encoding.
+    code = "import sys, trio; trio.run(trio.run_process, [sys.executable, '-c', ''])"
+    result = await run_process(
+        [
+            sys.executable,
+            "-X",
+            "warn_default_encoding",
+            "-W",
+            "error::EncodingWarning",
+            "-c",
+            code,
+        ],
+        capture_stderr=True,
+        check=False,
+    )
+    assert result.returncode == 0, result.stderr.decode()
+
+
 @background_process_param
 async def test_auto_update_returncode(
     background_process: BackgroundProcessType,