“Fat Old Sun” is a song by English rock band Pink Floyd, written and sung by David Gilmour. It appears on their 1970 album Atom Heart Mother, and was performed live in a greatly expanded form (often exceeding fourteen minutes), both before and after the album was released (10 October). Live performances of this song date back to 16 July 1970, but only Gilmour and keyboardist Richard Wright appear on the studio version.

This song was considered for the album Echoes: The Best of Pink Floyd (2001), as mooted by James Guthrie, the compilation's producer. "I wasn't allowed to put it on Echoes," Gilmour explained. "I was outvoted." The guitarist repeated this on Johnnie Walker's Radio 2 drivetime show in 2002. Atom Heart Mother is, consequently, unrepresented on Echoes.