Biologists with the University of Nevada, Reno say they're finding a growing number of the fish in the crystal clear lake.
Researcher Sudeep Chandra told KCRA-TV the discovery is especially worrisome because the fish eats a lot and excretes "lots of nutrients" that stimulate algae growth.
Some of the goldfish have grown to 18 inches and could eat smaller fish and create new competition for native trout.
Chandra says that with no prior studies on goldfish for guidance, researchers are catching the giant fish and bringing them back to their lab to study.
It's not clear how the goldfish got into Lake Tahoe, but it's believed to be from people dumping aquariums into the lake.