The Octagon built in 1834 is an historic octagonal building located at 888 Main Street on Roosevelt Island in New York City. It originally served as the main entrance to the New York City Lunatic Asylum which opened in 1841. Designed by Alexander Jackson Davis, the five-story rotunda was made of blue-gray stone that was quarried on the island. It is the last remnant of the hospital and after many years of decay and two fires was close to ruin. In 2006, a newly constructed residential building was built on the site, modeled on the original structure. It received LEED Silver status from the U.S. Green Building Council in 2008.
On March 16, 1972, it was added to the National Register of Historic Places.