Reid made 23 appearances for the Addicks this season, scoring six goals, but hadnt played since December 15th when he injured his hamstring against West Brom.
Charlton boss Alan Pardew said, "It was a difficult decision, but we have to look at ourselves and say 'are we going to let this affect us or will this make us stronger?'
"I feel we've coped well without him since he has been injured - he is still carrying an injury - but it was a tough call for all parties, and also Andy himself.
"I still think there are benefits to this deal for us as well as negatives, and the security of the club is key."
With Reid gone, Pardew is expected to sign Fulham winger Lee Cook on loan until the end of the season, and we will be here first with the news of his signature.
It's a very busy transfer deadline day at The Valley!!