Lee Sedol (Korean: 이세돌; born 2 March 1983), or Lee Se-dol, is a South Korean former professional Go player of 9 dan rank. As of February 2016, he ranked second in international titles (18), behind only Lee Chang-ho (21). His nickname is "The Strong Stone" ("Ssen-dol"). In March 2016, he played a notable series of matches against the program AlphaGo that ended in Lee losing 1–4.

Lee announced his retirement from professional play in November 2019, stating he could never be the top overall player of Go due to the increasing dominance of AI, which he called "an entity that cannot be defeated". Lee shared in a 2024 interview, "losing to AI, in a sense, meant my entire world was collapsing. ... I could no longer enjoy the game. So I retired."

Lee was born in South Korea in 1983. He is known as 'Bigeumdo Boy' because he was born and grew up on Bigeumdo Island. He studied at the Korea Baduk Association. He is the fifth-youngest (12 years 4 months) to become a professional Go player in South Korean history behind Cho Hun-hyun (9 years 7 months), Lee Chang-ho (11 years 1 months), Cho Hye-yeon (11 years 10 months) and Choi Cheol-han (12 years 2 months).

He ranks second in international titles (18), behind only Lee Chang-ho (21). Despite this, he describes his opening play as "very weak". In February 2013, Lee announced that he planned to retire within three years and move to the U.S. to promote Go. He plays on Tygem as "gjopok".

He is married to Kim Hyun-jin, and he has a daughter, Lee Hye-rim. His older brother Lee Sang-hoon [ko] is also a 9 dan professional go player.

This game was played between Lee Sedol and Hong Chang-sik during the 2003 KAT cup, on 23 April 2003. The game is notable for Lee's use of a broken ladder formation.

Normally playing out a broken ladder is a mistake, associated with beginner play, because the chasing stones are left weak. Between experts it should be decisive, leading to a lost game. Lee, playing black, defied the conventional wisdom, using the broken ladder to capture a large group of Hong's stones in the lower-right side of the board. This brought black's stones in the corner which were previously considered dead back to life. White ultimately resigned.

Starting 9 March 2016, Lee played a five-game match, broadcast live, against the computer program AlphaGo, developed by a London-based artificial intelligence firm Google DeepMind, for a $1 million match prize. He said “I have heard that Google DeepMind’s AI is surprisingly strong and getting stronger, but I am confident that I can win at least this time”. In an interview with Sohn Suk-hee of JTBC Newsroom on 22 February 2016, he showed confidence in his chances again, while saying that even beating AlphaGo by 4–1 may allow the Google DeepMind team to claim its de facto victory and the defeat of him, or even humanity. In this interview he pointed out the time rule in this match, which seems well-balanced so that both he and the AI would fairly undergo time pressure. In another interview at Yonhap News, Lee Se-dol said that he was confident of beating AlphaGo by a score of 5–0, at least 4–1 and accepted the challenge in only five minutes. He also stated "Of course, there would have been many updates in the last four or five months, but that isn’t enough time to challenge me".