A new species of monkeyflower, Mimulus sookensis, is described. This species is found throughout the southern portion of Vancouver Island, the Gulf Islands of British Columbia, the San Juan Islands of Washington state, the Willamette and Umpqua River Valleys in Oregon, and has been collected at one location in Mendocino County, California. Mimulus sookensis is a tetraploid species (n  =  28) derived from the predominately outcrossing Mimulus guttatus DC. (n  =  14) and the predominately self-pollinating Mimulus nasutus Greene (n  =  14). Mimulus sookensis is similar phenotypically to the small-flowered M. nasutus, but differs in chromosome number, height, and by a slightly more narrowed corolla tube than that of M. nasutus. It is commonly found on wet hillsides, seeps, cutbanks, and in roadside ditches, often co-occuring with M. guttatus but infrequently with M. nasutus.