The Goodnow Library is an historic public library building located at 21 Concord Road in Sudbury, Massachusetts. It is named for Sudbury-native John Goodnow II, who died in 1851 and left to the town of Sudbury a site for a library, $2,500 to build it, and $20,000 to buy books and to maintain it. Construction of the two-story octagon-shaped building began in 1862 and was finished in 1863. In the 1990s, the library was expanded to its present size, but the original octagon survives as a reading room.
On May 22, 2002, it was added to the National Register of Historic Places.