The talented West Ham United youngster will now remain with the Bantams until the start of January 2017.
The ball-playing central midfielder will certainly need no introduction to the vast majority of City supporters after a highly successful first loan spell with the club during the second half of last season.
Speaking about his popular return to the Northern Commercials Stadium, Cullen told bradfordcityfc.co.uk: "I really enjoyed my time at Bradford last season and it was a no-brainer really for me to return once I knew the club were interested in bringing me back.
"I am excited by the prospect of trying to push this club on again.
"Obviously in the summer I didn't know exactly what my situation was going to be for this year but as soon as I knew West Ham were allowing me to come (to Bradford), I was straight in the car to get up here.
"The fans, players and the staff were all brilliant with me first time around and I feel really comfortable at the club.
"West Ham were really happy with the way Bradford treated me and Reece (Burke) last season. This is a great club and I am really happy to be back."