The Josiah Mason Jr. House is a historic house at 11 Market Street in Cambridge, Massachusetts. It is a 2-1/2 story brick structure, three bays wide, with a side gable roof and a single end chimney. The entrance, located in the rightmost bay, is in a recess flanked by sidelight windows. The recess is framed by a Greek Revival surround with pilasters and entablature. Built in 1831, this house is a locally rare example of late Federal style architecture with a side-hall plan. Josiah Mason was a local merchant and politician who served as Cambridge selectman and in the state legislature.