Celebrated octogenarian architect Hugh Hardy — known for his work on theaters and public spaces, as well as restorations of landmarks like Radio City Music Hall and the Rainbow Room — is selling his condop apartment at 4 W. 21st St.

The 17th-floor unit has floor-to-ceiling windows in every room and is currently configured as a large one-bedroom.

There’s also an eat-in chef’s kitchen, spa-like bathrooms, hardwood floors and lots of storage space.

Building amenities include a gym, 2,500-square-foot landscaped terrace, children’s play area and on-site parking.

The listing brokers are Douglas Elliman’s Judith Auchincloss, Matthew Gulker and John Musso.

Hardy is also known for his angular, modernist rebuild of 18 W. 11th St.

That’s the West Village townhouse that was decimated when bombmakers from the radical leftist group Weather Underground accidentally blew themselves up while constructing an explosive in 1970.

Three members of the radical group died in the blast.