The pair will sit alongside Lord Lloyd Webber on the panel for the ITV1 programme, which aims to find a star to play Jesus in a brand new arena tour of Jesus Christ Superstar. A guest judge will also join the trio for each live broadcast.
"I'm gagging to find Jesus with the help of the Good Lord," said French in a statement. "I have actually been searching for him my whole life, and I can't believe that inside a few weeks we are going to find him. The fact that he is going to be a rock god is a massive bonus."
Donovan - who assisted at callbacks for the show in March - said: "We have unearthed some cracking talent so far and I'm really pleased I've had time to get to know the finalists as we've moved through the competition stages.
"It's an honour to sit by Andrew on the judging panel and I can't wait for the live shows to start."
Britain's Got Talent judge Amanda Holden, who will host the live stages of the show, added: "It's an absolute honour to be working with Andrew on Superstar.
"We've discovered some wonderful talent for the viewers, I already feel very protective of them, I'm like Mother Hen with all my boys."
The winner of Superstar will go on to star alongside comedian and musician Tim Minchin, former Spice Girls singer Melanie C and BBC Radio 1 breakfast show host Chris Moyles in an arena tour which begins on September 21.
Superstar will launch this summer on ITV1.