diff --git a/Misc/NEWS.d/next/Library/2020-11-10-14-27-49.bpo-42237.F363jO.rst b/Misc/NEWS.d/next/Library/2020-11-10-14-27-49.bpo-42237.F363jO.rst
new file mode 100644
index 000000000000000..50cab6e1f11f847
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-11-10-14-27-49.bpo-42237.F363jO.rst
@@ -0,0 +1 @@
+Fix `os.sendfile()` on illumos.
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index 726e3723f99d396..3b33186c59521cb 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -9281,9 +9281,27 @@ posix_sendfile(PyObject *self, PyObject *args, PyObject *kwdict)
     if (!Py_off_t_converter(offobj, &offset))
         return NULL;
 
+
+#if defined(__sun) && defined(__SVR4)
+    // On illumos specifically sendfile() may perform a partial write but
+    // return -1/an error (in one confirmed case the destination socket
+    // had a 5 second timeout set and errno was EAGAIN) and it's on the client
+    // code to check if the offset parameter was modified by sendfile().
+    //
+    // We need this variable to track said change.
+    off_t original_offset = offset;
+#endif
+
     do {
         Py_BEGIN_ALLOW_THREADS
         ret = sendfile(out, in, &offset, count);
+#if defined(__sun) && defined(__SVR4)
+        // This handles illumos-specific sendfile() partial write behavior,
+        // see a comment above for more details.
+        if (ret < 0 && offset != original_offset) {
+            ret = offset - original_offset;
+        }
+#endif
         Py_END_ALLOW_THREADS
     } while (ret < 0 && errno == EINTR && !(async_err = PyErr_CheckSignals()));
     if (ret < 0)