The Scot only took up the reins with the Hammers in November, penning a short-term contract after taking over from the sacked Slaven Bilic.
And despite West Ham securing their top-flight status with two games to spare, the English outfit have opted against keeping on the former Manchester United and Everton coach for the long-term.
"When David and his team arrived, it was the wish of both parties that the focus be only on the six months until the end of the season, at which point a decision would be made with regards to the future," said West Ham joint-chairman David Sullivan, as quoted by BBC Sport.
"We feel that it is right to move in a different direction. We aim to appoint a high-calibre figure who we feel will lead the club into an exciting future for our loyal supporters within the next 10 days."
Moyes' assistants Alan Irvine, Stuart Pearce and Billy McKinlay have also left with immediate effect.