We describe Myotis midastactus sp. nov. from the Bolivian savanna on the basis of differences in fur color, and cranial and external features that unquestionably distinguish it from all other Neotropical Myotis. This new species is morphologically allied to M. simus Thomas, 1901 and other species in the M. ruber group. Myotis midastactus is endemic to Bolivia, where it occurs with 6 congeners—albescens, dinellii, keaysi, nigricans, riparius, and oxyotus. Previously identified as M. simus, M. midastactus is in syntopy with M. nigricans and M. riparius in the department of Santa Cruz, and there is no evidence that true M. simus occurs in Bolivia.