The Sulphur-rumped Myiobius lives in the lower and middle levels of humid forest. Males have a dark olive back and pale yellow rump; the yellow crown patch is often hidden.
An adult male is mostly dark olive above, with a pale yellow rump. The wings and tail are black; the throat, eye-ring, and auricular stripe are buffy white; the breast and flanks are ochraceous; and the belly is pale yellow. A large erect yellow crown patch is often hidden. Females have little or no crown yellow. Juveniles are browner above, lack the yellow crown patch, and have dusky feather fringes that make the breast and flanks look mottled.
It occurs in the lower and middle levels of humid forest, tall second-growth, and gallery forest. The Costa Rican account describes it as solitary and not forming pairs.
An extinction-risk assessment has not been added yet.