Assistant manager Dean White and first-team coach Bob Booker have also left the club, with Martin Hinshelwood taking over as caretaker manager.
Slade was appointed in March on a contract until the end of the season and was rewarded with a two-year deal after ensuring the club avoided relegation.
However, Albion are hovering just one place above the drop zone following Saturday's 3-3 draw at home to Hartlepool and chairman Tony Bloom felt it was the right time for a change.
Bloom told the club's official website: "It is not a decision we have taken lightly and one taken with a heavy heart.
"Russell is a good man, which made it an even harder decision to take, but it is one which has been made in the club's best interests.
"Like all Albion fans I am extremely grateful for Russell's achievements at the end of last season, as he kept us in League One against the odds."