The mammal, which resides at an animal sanctuary in Cornwall, will have his portions rationed to prevent his weight increasing any further.
Metro quotes animal care assistant Jenny Lewis as saying: "He's such a lovable character I haven't been able to resist giving him the odd extras at feeding time.
"From now on he gets only his allotted 4.4lb (2kg) of fish for breakfast and 3.3lb (1.5kg) for supper.
"And as well as his regular morning training session, there will be extra enrichment sessions every day where I have to make him work for his food."
The seal will also be weighed monthly to monitor his progress.