The striker, 28, has been out with a cruciate ligament injury since September and has yet to play under Rovers boss John Ward.
Harrold told the club's official website: "I'm really delighted to have been offered a new contract. It's something that's always on your mind when you get injured, but I've been speaking to the chairman and the manager about a new deal for a while and there weren't any problems at all and I'm now looking forward to being part of a winning side.
"I must add that I am very grateful to the chairman and the manager for sticking by me all the time I have been injured and I hope I can repay them for the faith they have shown in me."
- Sports & Recreation