Seafood Buffet Nanda (難陀)
Nanda is a sure bet when you want to try a little bit of everything. The all-you-can-eat buffet is one of the best introduction to the food culture of Hokkaido there is. You can have seafood sashimi as well as grilled seafood. Tables are equipped with roasters on which you grill your items yourself. This is a particularly fun experience for those who have never had this kind of dining style. Not only do they serve seafood, but also some fine wagyu beef as well. Lunch prices for a 70-minute all-you-can-eat course are ¥3,700 per adult and ¥4,700 with the all-you-can-drink alcohol option. Dinner prices for 100-minute courses are ¥4,780 per adult and ¥5,980 with the all-you-can-drink alcohol option.
Located a 5-minute walk from the Susukino Station.
Hakodate Uni Murakami (函館うに むらかみ)
This restaurant is famous for serving one of the most amazing uni (sea urchin) donburi (donburi means ingredients placed on rice). Uni is rich, smooth and has a sea taste many people absolutely love. It's one of Hokkaido's famous delicacies. You normally don't eat so much of it in one bowl so if you are a fan of uni, you'll be in heaven. The uni donburi is about ¥4,000.
You can also have a mixed bowl as well if you want more variety. The restaurant is located near the JR Sapporo Station.
Kaiyotei serves sophisticated courses with lots of variety of seafood at a reasonable price (between ¥3,000 to ¥5,000) considering the quality of the products. The restaurant is famous for its sashimi squid and its crab so make sure to try those if you aren't taking a course in which you will be served some.
Kaiyotei is at a 4-minute walk from the Susukino Station.
Kita No Gurumetei (北のグルメ亭)
This restaurant is located in the Sapporo Wholesale Market, a popular market in Sapporo with some 60 establishments comprising restaurants and wholesale stores. Kita Gourmetei serves some of the most tantalizing kaisen don (seafood toppings on a bowl of rice) you'll ever see. They contain all of the specialties of Hokkaido in one bowl (crab, sea urchin, salmon roe, seasonal fish, etc.). Expect to pay about ¥3,000 for the best items on the menu. A little pricey compared to other places at the Sapporo Wholesale Market but it is well worth it!
Located at a 5-minute walk from the Nijuyonken Station.
Kaki To Shunsenryori To Osoba (かきと旬鮮料理とおそば)
This is a wonderful traditional restaurant specializing in oysters. You can either sit at the counter or at tatami seats or at horigotatsu (low tables with a whole underneath them to place your legs). All dishes are sublime, and there are so many ways to have them (sauteed in butter, raw, in a hot pot, deep fried etc.). If you can't decide what to have, you might want to try the kaki course (oyster course, ¥5,700) which comes with several dishes in which the oysters are prepared differently. Hiraku specializes in soba (buckwheat noodles) as well, so you might want to try that too. They also have a good selection of Japanese alcohols to pair with your meal.
Located near the Nishi Yon Chome Station.
All-You-Can-Eat Crab Buffet Restaurants
Among all the seafood in Hokkaido, crab is arguably the most popular. There are many kinds of Hokkaido crabs to discover, each being in-season at slightly different times. If you want to eat crab and lots of it in Sapporo, you should refer to the link below for a great selection of crab all-you-can-eat buffet restaurants.
Sapporo Wholesale Market
A spot that is definitely on every Japanese tourist's radar map when it comes to eating great seafood at a cheap price in Sapporo is the Sapporo Wholesale Market (also known as Sapporo Jogai Ichiba). The seafood and fresh produce market is particularly well-known for its many small restaurants serving outstanding kaisen don (variety of seafood as topping for a bowl of rice). See the link below for some of the best places at that market.