Mimulus hymenophyllus is a species of flowering plant in the lopseed family known by the common names thinsepal monkeyflower and membrane-leaf monkeyflower. It is native to Hells Canyon on the border between Oregon and Idaho in the United States. It has also been reported from Montana. This Mimulus is considered to be a member of the Snake River Clade (with Mimulus ampliatus and M. patulus) of the Mimulus moschatus alliance.