Muddy Fork is a tributary, about 6 miles (10 km) long, of the Sandy River in the U.S. state of Oregon.

It joins the Sandy River in Old Maid Flat, about 5 miles (8 km) northeast of Rhododendron.

Muddy Fork begins at 5,890 feet (1,800 m) above sea level at the foot of Sandy Glacier in the Mount Hood Wilderness of the Cascade Range.

It then flows between Yocum Ridge, which is to the left, and Bald Mountain to the right before being crossed by the Pacific Crest Trail near the eastern end of Old Maid Flat.

[3][4] Although Muddy Fork is closed to salmon fishing, the stream has wild cutthroat trout "of fair size".