The former West Bromwich Albion midfielder left the Killers as a free agent last summer to join Brendan Rodgers’ side on a two-year deal.
Mulumbu’s move to Celtic Park in August did not meet expectations as he was limited to three appearances across all competitions – one game in the Premier League and two in the Uefa Europa League.
In his bid to play regularly, the former PSG and Norwich player is back at the Ayrshire outfit where he played 20 matches last season.
During his first stint at Kilmarnock, the 32-year-old helped the club achieve club-record total points in the 2017-18 campaign and scored the decisive goal in their win over Celtic last February.
“Youssouf Mulumbu has sealed a return to Kilmarnock, signing a six-month loan deal from Celtic,” read the club statement.
“A fans’ favourite during his first spell with Steve Clarke’s side in 2017/18 season, the Congo international helped Killie achieve a club-record points total.
“Everyone at the club would like to welcome Youssouf back to Scotland’s oldest professional football club.”
Kilmarnock are third in the Scottish top-flight standings, level on points with Rangers [45 points from 23 matches].