diff --git a/Misc/NEWS.d/next/Library/2026-07-22-10-30-08.gh-issue-154435.xCxm0T.rst b/Misc/NEWS.d/next/Library/2026-07-22-10-30-08.gh-issue-154435.xCxm0T.rst
new file mode 100644
index 00000000000000..b468e929b1ca18
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-07-22-10-30-08.gh-issue-154435.xCxm0T.rst
@@ -0,0 +1,3 @@
+Fix :func:`os.posix_fadvise` and :func:`os.posix_fallocate` on DragonFly BSD:
+they raised :exc:`OSError` with a meaningless error code,
+because these functions return -1 and set ``errno`` there.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index d4390370cc2f75..d995fefe1adbe8 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -13475,6 +13475,10 @@ os_posix_fallocate_impl(PyObject *module, int fd, Py_off_t offset,
         Py_BEGIN_ALLOW_THREADS
         result = posix_fallocate(fd, offset, length);
         Py_END_ALLOW_THREADS
+        // DragonFly BSD returns -1 and sets errno.
+        if (result == -1) {
+            result = errno;
+        }
     } while (result == EINTR && !(async_err = PyErr_CheckSignals()));
 
     if (result == 0)
@@ -13522,6 +13526,10 @@ os_posix_fadvise_impl(PyObject *module, int fd, Py_off_t offset,
         Py_BEGIN_ALLOW_THREADS
         result = posix_fadvise(fd, offset, length, advice);
         Py_END_ALLOW_THREADS
+        // DragonFly BSD returns -1 and sets errno.
+        if (result == -1) {
+            result = errno;
+        }
     } while (result == EINTR && !(async_err = PyErr_CheckSignals()));
 
     if (result == 0)