Tylodina fungina, commonly known as the yellow umbrella slug or the mushroom sidegill, is a species of sea snail or false limpet, a marine opisthobranch gastropod mollusk in the family Tylodinidae.

[2] The yellow umbrella slug is found in the eastern Pacific Ocean, from Cayucos, San Luis Obispo County, California, to Banderas Bay, Jalisco, Mexico,[2] as well as the Galapagos Islands.

[3] They inhabit the intertidal zone, and are often observed at depths of 10–20 feet underwater.

Their other common name, yellow umbrella slug, is shared with their relative, Tylodina perversa, which lives in the northeastern Atlantic Ocean.

Tylodina fungina is specialized to feed on the yellow sulfur sponge, Aplysina fistularis, according to the in situ observations on the Pacific coast of Mexico.