SR 15 ends at the Michigan state line just north of Pioneer, Ohio, where it becomes M-99. The roadway travels northward from the state line on Pioneer Road, through mostly agricultural areas of Hillsdale County before reaching a junction with M-34 just west of Osseo. The trunkline turns westward on Hudson Road, which curves to the north around Baw Beese Lake just southeast of Hillsdale. M-99 passes through Hillsdale on a northwest course on Broad Street and Carlton Road, passing just a few blocks to the west of Hillsdale College. After leaving town, the road bends to the north as Olds Street as it runs parallel to the St. Joseph River to Jonesville. In that community, M-99 intersects US 12. The two highways run concurrently through Jonesville for less than half a mile (0.8 km) before M-99 returns to its northwesterly course. It leaves the town toward Litchfield continuing parallel to the river.