Noryangjin is Seoul's major wholesale fish market, occupying a large building south of the Han River near Noryangjin Station. The best time to visit is in the early hours — 3am to 6am — when the overnight tuna and fish auctions happen and the market is at its most chaotic and theatrical. The regular retail market operates from around 6am and is accessible to anyone willing to navigate the stalls.
The process is to buy fish from the vendors on the market floor, then take it upstairs to one of the restaurants that will cook it to order — raw (hoe), grilled, or in a stew. The quality is excellent and the prices are reasonable by Seoul standards. A meal of sliced raw fish with side dishes and drinks for two should cost around 50,000-80,000 won depending on what you choose. A good late-night option since the market and upstairs restaurants operate round the clock.