High temperature ferromagnetic In1−xMnxSb semiconductor alloys with a Curie temperature (TC) above 400 K were investigated. Alloys with x ranging from 0.08 to 0.22 deposited by metalorganic vapor phase epitaxy were examined. X-ray diffraction indicated alloys are primarily two phase consisting of a zinc blende InMnSb solid solution and hexagonal MnSb precipitates. Transmission electron microscopy analysis confirmed the presence of hex-MnSb nanoprecipitates as well as the presence of the additional minority phases Mn3Sb, metallic Mn, and MnAs1−xSbx. Magnetization measurements indicate that the alloy films are ferromagnetic, showing clear hysteresis in field dependent measurements from 5 to 400 K. Magnetization values as high as 47 emu/cm3 for an alloy with x = 0.22 were measured at room temperature. Irreversibility is observed between field-cooled and zero-field-cooled magnetization curves that is attributed to inhomogeneous magnetic order arising from randomly distributed ferromagnetic nanoprecipitates. Temperature dependent magnetization indicates at least two magnetic phases are present, one with a nominal TC of 300 K that is attributed to MnAs1−xSbx nanoprecipitates which form at the GaAs substrate interface, and a second with a TC > 400 K that is attributed to hex-MnSb nanoprecipitates and to the InMnSb matrix. The majority of magnetization arises from the high TC phase, where fitting the temperature dependent magnetization curve with a Brillouin function indicated a TC of 570 K. Magnetization measurements indicate that there is an interparticle magnetic interaction. Large saturation magnetization at 300 K and TC of 570 K make these multiphase InMnSb epitaxial films excellent candidates for ferromagnetic layers in semiconductor spintronic devices that operate at room temperature.