Iwelumo has signed on a free transfer after his contract at Colchester United expired. The 28-year-old Scottish international scored 18 goals in the Championship last season and scored 35 goals in two years at Layer Road.
Manager Alan Pardew said, "Luke is an exciting young player, and perfectly fits the mould of a Charlton player.
"He's diligent, intelligent, quick and a player that is going to get even better.
"We've beaten off stiff competition from both Championship and Premiership clubs to sign him - and I'd like to thank the board for backing my judgement on him."
"Chris has a great goalscoring record, and the fact he is a free transfer because he's out of contract represents a fantastic bit of business for the club," said Pardew.
"He's at the prime of his career and is a big target man, and in the Championship you have to have a player of that type.
"He's got better every year and I hope that improvement continues under myself and Phil Parkinson here at Charlton."
Pardew added: "I'm sure the signings of Luke and Chris will excite Charlton fans and I'm hoping to announce more as we go through the summer."