Arsenal Arsenal Give Us A Song
The 19-year-old has played twice in the Premiership this season and featured in the Gunners' Carling Cup run.
'We've been tracking Alexandre for some time and I'm positive he'll be a great addition to our midfield,' said Charlton boss Alan Pardew.
'Arsenal received interest from several top Premiership clubs, and I'm happy Alexandre has chosen us.'
The Cameroon youngster, who scored his first Arsenal goal in the 6-3 Carling Cup win at Liverpool, is Charlton's fourth transfer window signing.
Zheng Zhi, Ben Thatcher and Madjid Bougherra have already arrived at the club during January.
Song, the nephew of former Cameroon international Rigobert, initially joined Arsenal on loan from French club SC Bastia in July 2005.
Arsenal made the move permanent - and Gunners boss Arsene Wenger recently admitted that he would consider loaning out the player.
'The only one who may go out on loan is Song, because I want him to play somewhere,' said Wenger last week.
'In central midfield we are a bit congested, and he is ready to play.'
Song has also appeared in the Champions League, as an 89th minute substitute in the 2-1 win over Porto on 26 September.
The midfielder is currently in Congo for the African Youth Championship.
Arsenal fans don't seem to rate the youngster so far and the player is hoping to impress his parent club while at The Valley.