Cullen has made 23 appearances for the Bantams since signing on an initial five-month deal in August.
The 20-year-old also played in 17 games for the club last season after joining them on loan in February.
"Everything about the club - the staff, the players, the fans - they have all been brilliant with me," he said.
League One side Bradford City have extended West Ham midfielder Josh Cullen's loan spell at the club until the end of the season.