At age of 12, Lee caught the attention of FC Barcelona after he finished as the top scorer in the 2010 Danone Nations Cup, one of the most prestigious youth football tournaments. He joined Barcelona's youth academy, La Masia, and scored 39 goals with 29 appearance in his first season. He was also named the best player in four youth tournaments, and was nicknamed the "Korean Messi". However, he was banned from playing in the regular season for three years until his 18th birthday by transfer regulations of FIFA. On 13 March 2016, he made his senior debut for the B team.
Club career
Verona
On 31 August 2017, Lee joined Italian club Hellas Verona on a four-year deal, for a fee of €1.5 million. Barcelona retains the option to repurchase him until 2019 for an undisclosed fee. On 24 September 2017, he made a Serie A debut for Verona in the second half of the game against SS Lazio. On 6 May 2018, he scored his first Serie A goal in an away match against Milan but his team lost 4–1. Verona was relegated to Serie B by finishing their league as 19th place. In the 2018–19 season, he recorded one goal and two assists with 27 appearances and his team was promoted to the Serie A by winning the promotion play-offs.
Sint-Truiden
On 30 August 2019, Lee joined Belgian First Division A side Sint-Truiden. He was excluded from the list in twelve consecutive matches and this caused much controversy in South Korea. On 29 November 2019, his name was on the list for the first time against Genk but didn't appear. On 26 December 2019, he made his Belgian league debut against Waasland-Beveren. He finished his first season with only four appearances according to early termination of the Belgian league, caused by the COVID-19 pandemic.
Lee made his international debut as a part of South Korea's under-16 team competing for the 2014 AFC U-16 Championship qualification. He scored four goals against the team from Laos. In April 2014, he was a part of the South Korea under-16 team that was the runner-up at the Montaigu Tournament. He scored two goals in the competition. In September, he led South Korea to another runner-up finish, this time at the 2014AFC U-16 Championship. He scored winning goals against Malaysia and Thailand in the group stage. He scored two goals against Japan in the quarter-final and led team's 2–0 win. He received the ball at around the halfway line, destroyed Japan's defense by dribbling alone for approximately 50 meters and struck his second goal. He recorded one goal and four assists against Syria in the semi-final and South Korea won 7–1. He was named the MVP of the tournament, although his team lost to North Korea in the final.
In 2015, Lee was the youngest member of South Korea's under-18 team that participated in the :ko:수원 JS컵|Suwon JS Cup, hosted by Park Ji-sung. He also participated in the :ko:수원 컨티넨탈컵|Suwon Continental Cup, an under-17 competition, and scored two goals against Croatia. At the 2015 FIFA U-17 World Cup, South Korea advanced to the knockout stage by defeating Brazil and Guinea in the group stage. However, Lee missed a crucial penalty kick against Belgiumin the round of 16 and South Korea eventually lost 2–0. He shortly returned to South Korea afterwards to train with the second division side Suwon FC until January 2016, when he turned 18 and became able to participate in official matches for FC Barcelona again.
Lee participated in the :ko:아디다스 U-20 4개국 축구대회|Adidas U-20 Tournament to prepare the 2017 FIFA U-20 World Cup. He scored two goals against Zambia, including a chip shot. He was named in the under-20 squad for the 2017 U-20 World Cup, hosted in South Korea. He scored the winning goal and assisted team's second goal in the first match against Guinea. He scored the opening goal by dribbling alone for 40 meters against Argentina, and this goal was ranked fourth in top 10 goals of the tournament. South Korea advanced to the round of 16 but lost to Portugal.
Lee was called for senior national side for the first time before 2018 FIFA World Cup when in May 2018 and was named in South Korea's preliminary 28-man squad. On 28 May 2018, he has played a big part in South Korea's 2–0 victory over Honduras in Daegu Stadium. He played a short pass, a bold dribble, and witty play in midfield. He also assisted Son Heung-min's starting goal. He participated in the 2018 World Cup and appeared in two matches as a substitute.
Lee was also named in the 2018 Asian Games in Jakarta-Palembang, Indonesia led by coach Kim Hak-bum. At 23 August, when Korea was playing against Iran, Lee Seung-woo scored the first goal of the Asian Games, leading to the victory of the team by 2–0. At the semi-final against Vietnam, Lee scored two goals and led the team to the final by 3–1. At the final against Japan, Lee started the game as a sub. During the second half, Lee switched with Kim Jung-min, but both teams could not score in the first 90 minutes. In extra time, Lee scored the opening goal and contributed to a 2–1 victory. Lee and his teammates won the gold medal in the Asian Games.
