Last week Smith was interviewed by Jo Wiley on BBC Radio 2 and said he was "definitely not" working on the film.
He has now revealed in an interview on BBC Radio 1 that he recorded the track, called Writing's on the Wall, in January. It will be released on 25 September.
Smith recorded the song with fellow Grammy award winner Jimmy Napes, who worked with him on the number one hits Stay With Me and La La La. He is the first British male solo artist to record the James Bond theme tune since 1965.
Ellie Goulding, Radiohead and Lana Del Ray had also been in the running to record the theme for Spectre, which is out next month.
Last month, the bookmaker William Hill suspended betting on the identity of the next Bond theme singer after a customer tried to put a £15,000 bet on Radiohead having recorded the song.