Female pensioners taking the advice in a trial doubled the rate at which exercise improved their muscles. A bigger study involving men and women is under way.
Experts believe the anti-inflammatory properties of omega-3 fatty acids in fish oil counteract the effects of ageing that weaken the body.
Age-related loss of muscle mass seriously affects around 20% of those aged 50 to 70 and half aged 80. While exercise helps, ageing muscles do not respond as well as young ones.
"We're trying to make older muscle adapt like younger muscle. That's where we think fish oil can come in," said lead scientist Dr Stuart Gray from Aberdeen University.
Fourteen elderly women took part in last year's study. Half took four grams of fish oil a day for a month, while the others took olive oil. They attended gym twice a week.
"In the placebo group, muscle growth improved by 11% in response to exercise, but the fish oil group improved strength by 20%," Dr Gray said.