Queens Park Rangers striker
The 32-year-old appears to have no future at Loftus Road after he was left out of Rangers' 25-man Premier League squad, and he will go straight into Charlton's squad for Tuesday's (Wednesday AEST) visit of Watford.
Powell told the club's official website: "We have lost Yann long-term, and Rob will give us that strength and that Championship know-how and experience up top.
"Things haven't worked out for him at QPR but he's here for three months and I'm sure he will do very well here, enhance our squad and remind people that he's a good player.
"We've seen him recently and I really hope he comes in and does well for us."
Atletico Madrid executive Gil Marin has admitted the club could replace star striker Radamel Falcao with Sevilla's Alvaro Negredo.