Diamond Lake is a lake in the southern part of the U.S. state of Oregon. It lies near the junction of Oregon Route 138 and Oregon Route 230 in the Umpqua National Forest in Douglas County. It is located between Mount Bailey to the west and Mount Thielsen to the east; it is just north of Crater Lake National Park.

Diamond Lake was named for John Diamond, for whom Diamond Peak is also named. He discovered the lake in 1852 while on the summit of Diamond Peak. Diamond was a pioneer settler of Coburg, Oregon who was part of a party that was opening a road between the Middle Fork Willamette River and Idaho as an immigration route.

The post office of Diamond Lake, Oregon was established in 1925 and ran until 1956, when it was changed to a summer-only office.

In 1992, non-native tui chub were detected in the lake. These fish, presumed to have been introduced illegally, grew quickly in population, and had a strong negative impact on the lake's ecology. The lake was previously a "showcase trout fishery," but its water became murky, trout declined, and the insect population dropped dramatically. This had also happened at the lake in 1954.

The Oregon Department of Fish and Wildlife poisoned the lake with Rotenone in 2006 to kill off the tui chub and restore balance to the ecosystem. The project was more successful than anticipated; in 2007 dramatic improvements were noted.