The Pleasant Street Historic District is a predominantly residential historic district at 187—235 Pleasant Street in Marlborough, Massachusetts. It is a cohesive collection of 8 residences and one school building that are representative of upper class housing on large well-proportioned lots. The oldest house in the district was built c. 1865, the newest c. 1924. A number of 19th century houses were restyled during the 1920s. The Immaculate Conception School was built in 1961, and does not contribute to the area's historic significance. The district was added to the National Register of Historic Places in 2001.