Sometimes — with care
Small occasional amounts of cooked or canned-in-water tuna won’t harm most dogs — but tuna’s mercury content means it should never be a regular item.
Tuna is a large, long-lived predator fish, which means it accumulates significantly more mercury than smaller fish like salmon, sardines or whitefish. An occasional bite of water-packed tuna is fine; regular tuna meals risk slow mercury build-up, and dogs are more sensitive to it than humans per kilo. Oil-packed tuna adds unnecessary fat; brine-packed adds salt; and seasoned tuna salads add onion and mayo. Raw tuna (sushi-grade or not) is best avoided for parasites and thiaminase concerns.
A teaspoon to a tablespoon of water-packed tuna mixed into food, no more than once a week or so, depending on size. If your dog snagged a whole tuna sandwich, the bigger worry is usually the onion in it — check ingredients and call your vet if alliums were involved.
Sardines in water (small fish = low mercury) or plain cooked salmon are better omega-3 sources for dogs.
Related: Can cats eat tuna? · Can dogs eat shrimp? · Can dogs eat eggs?
This article is general information, not veterinary advice. If your pet has eaten something potentially harmful or shows symptoms, contact your vet or an emergency clinic immediately. Full disclaimer.