McCormack began his career at Preston and had loan spells at Leyton Orient and Motherwell as well as Southend.
"I'm really pleased to have Alan. He fits the bill in terms of his enthusiasm and his hunger to join Charlton," boss Phil Parkinson said.
"Anyone I bring into this club, whether it's a member of staff or a player, has got to relish the challenge.
"He's a player I've watched many, many times and I always come away admiring his performance.
"We look forward to working with him."
McCormack follows the arrival of former Norwich defender Gary Doherty to The Valley.
Source: BBC Sport
Source: BBC Sport