Forster, who rejoined the club in July after 22 games on dual registration at Haven earlier in his career, replaces sacked James Coyle.
He began his career at Saints, winning the academy Grand Final there, but was limited to five senior appearances.
"This appointment will lift everybody," general manager Amanda Hewer said.
The ex-Salford forward takes charge after a season which saw Haven relegated from the Championship, despite winning three of the final five games in the Super 8s.
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Whitehaven have appointed former St Helens prop Carl Forster as player-coach at the age of 24, the youngest in the professional game.