Saint-Malo's food is seafood-driven: fresh oysters from the nearby beds at Cancale (15km east), mussels moules marinières, sole meunière, and the Breton staple of galettes de sarrasin (buckwheat crepes) served with seafood fillings. The Intra-Muros has a good concentration of restaurants, most on the rue Jacques Cartier and the streets east of the cathedral.
The crêperies are ubiquitous and variable in quality — the best stick to Breton tradition and use stone-ground buckwheat flour. The tourist-facing places near the main gate tend to be overpriced; walk two streets further into the old town for better value. Cancale, a short drive or bus ride east, has a famous oyster market on the seafront where you can buy oysters direct from the producers.