Charlton chief executive Katrien Meire says it was "a very difficult decision" to sack head coach Bob Peeters.
Peeters took charge at The Valley last May and the Addicks started the season strongly with an 11-match unbeaten run.
But the Belgian was dismissed on Sunday following a run of one win in 12 Championship matches.
"The disappointing nature of performances and results in recent matches forced us to look at the situation," she told the club website.
"We had been very supportive of Bob until the very last moment
|Charlton's record under Bob Peeters (all competitions)|
"With the January transfer window open it is an important time of the season. We feel this is the right decision."
Charlton's early-season promise has given way to a slump, with the Addicks' last league win coming against Reading on 8 November as well as early exits in both the League Cup and the FA Cup.
Meire said it was important for the club "to make sure this next appointment is the right one" with Charlton having seen Chris Powell, Jose Riga and now Peeters depart as manager since owner Roland Duchatelet took charge of the club in January 2014.
"I understand that this club needs and deserves a head coach who can plan and take charge for the long-term," she added.
"I realise this decision has taken some by surprise, but there is still a lot to play for this season and we want to try to ensure the 2014-15 campaign ends as successfully as it started."
First-team coach Damian Matthew and goalkeeping coach Ben Roberts have been placed in caretaker charge of the south east London club.