Nijo Market has been selling Hokkaido seafood since 1903, occupying two covered lanes in the centre of the city between Odori Park and the Sapporo TV Tower. The fishmongers sell crab, sea urchin, salmon roe, scallops, and Pacific saury from the cold waters around Hokkaido, and the quality is noticeably higher than comparable markets in warmer parts of Japan. The cold-water fisheries around Hokkaido produce shellfish and sea urchin that are difficult to match. Several market restaurants serve kaisendon — rice bowls loaded with fresh seafood — at prices that are fair given what you're getting.
Go for breakfast or early lunch when the selection is widest and the fish is freshest. By early afternoon some stalls have sold through their best stock. The market is smaller than Tsukiji and easier to navigate; you won't get lost, and the vendors are accustomed to visitors without Japanese. Buying to take away is possible but the freshness argument applies more to eating on the spot. The crab preparation services — where a vendor will select, cook, and section a crab for you to eat at a standing counter — are the best value per experience in the market.