The Irishman is to continue at Valley Parade until the end of the season.
The Bantams boss is quoted by BBC West Yorkshire Sport as stating:
"We've signed Josh Cullen until the end of the season on loan.
"I'm delighted we've agreed that with West Ham.
"He's a winner"
This is a great deal for Bradford, who initially signed Cullen on loan for just a month in February. But he has obviously done enough to impress and he will now be tasked with helping Bradford in their hunt for League One promotion.
The Republic of Ireland under-21 international has already played six times for Bradford since making the move to Yorkshire and is not alone at the club. Fellow Hammer Reece Burke is also on loan at Valley Parade.
This has been something of a breakthrough campaign for Cullen. He made his West Ham debut against FC Lusitans in the Europa League qualifying in the summer.
He went on to make his Premier League debut against Liverpool back in August.
He will be hoping to continue his good form at Bradford for the remainder of this season, with the hope of becoming a West Ham regular.