King scored 47 goals for Gillingham over three seasons before joining Forest for £950k, but he has not featured in new manager Megson's plans.
"When I learnt Kevin Blackwell wanted me at Leeds I did not hesitate," King told the official Leeds website.
"It's a massive club and one which should be in the Premiership. When I get my chance I hope to make the most of it and get some goals."
Blackwell himself added: "A striker gives you another option sometimes, we're in with a chance of pushing into the play-offs and I might as well go for it.
"If you sign a defender people say you're being negative, but with Marlon I know we've got a good option there."