"I'm a Believer" is a song written by Neil Diamond and recorded by American band the Monkees in 1966 with the lead vocals by Micky Dolenz. The single, produced by Jeff Barry, hit the number-one spot on the U.S. Billboard Hot 100 chart for the week ending December 31, 1966, and remained there for seven weeks becoming the last number-one hit of 1966 and the biggest-selling single for all of 1967.

Source: Wikipedia