McElroy Octagon House, Historic octagonal residence in Cow Hollow, San Francisco, United States.

The McElroy Octagon House is an eight-sided wooden building on Gough Street in San Francisco, with two floors joined by a central winding staircase. Rectangular windows run along the sides of the structure, giving the building its distinctive eight-sided outline that sets it apart from the surrounding neighborhood.

William McElroy had the house built between 1860 and 1861, following ideas put forward by writer Orson Fowler, who had argued for octagonal homes in his widely read book from 1848. The style attracted followers across the United States for a time before falling out of fashion.

The house is also called the Colonial Dames Octagon House after the National Society of the Colonial Dames of America in California, which bought it in 1951 and opened it as a museum by 1953. The name on the plaque by the porch, "McElroy Octagon House" with the date 1844, recalls the family of William C. McElroy, a wood miller who built it.

The house stands at 2645 Gough Street, at the corner of Union Street in Cow Hollow, and is open to the public for tours. The original 1860s site lies directly across the street, where condominiums were built in 1955. Look for the plaque reading "McElroy Octagon House" and "1844" near the columned porch.

The house was moved across Gough Street in 1951, and condominiums were built on its original lot in 1955. It is one of only three octagon houses left in San Francisco, alongside the Feusier Octagon House and the Marine Exchange Lookout Station at Land's End.