Matsushita, whose first experience in swimming came at age one, won his first swimming competition in junior high school.[6] In 2022, he competed at the Junior Pan Pacific Swimming Championships and won four medals, including individual medals in the 200m individual medley (silver) and 100m butterfly (bronze).[7] Later that year, he competed at the 2022 FINA World Junior Swimming Championships and won silver in the 200m individual medley.[2] The following year, he was selected for the 2023 World Aquatics Junior Swimming Championships, where he won the gold medal in the 400m individual medley while setting the championship record in the event with a time of 4:10.97.[5][6]
In March 2024, Matsushita became the first Japanese swimmer to qualify for the 2024 Summer Olympics, winning gold at the Japanese Olympic trials in the 400m individual medley while beating out previous Olympic medalists Tomoru Honda and Daiya Seto.[4][5][8] At the Olympics, Matsushita was the youngest participant in the event but reached the finals, where he won the silver medal, only behind gold medalist Léon Marchand of France.[5][9] He became the youngest medalist in the 400m individual medley since 1972.[10]
^"宇都宮出身の競泳松下選手 パリオリンピック400メートル個人メで決勝進出 地元住民が声援" [Utsunomiya-born swimmer Matsushita advances to finals in 400m individual event at Paris Olympics; local residents cheer him on]. shimotsuke.co.jp (in Japanese). 28 July 2024.
^ abc"【競泳】高校3年の松下知之、競泳ニッポン一番乗りで初五輪「気持ちで押し切った」" [[Swimming] Tomoyuki Matsushita, a third-year high school student, is the first Japanese swimmer to compete in his first Olympic Games. "I pushed through with all my heart."]. Nikkan Sports (in Japanese). 19 March 2024.
^"競泳 五輪代表内定の松下が練習公開 "1番をねらい勝負挑む"" [Swimming: Olympic representative candidate Matsushita opens practice to the public "Aiming for first place"] (in Japanese). NHK. 14 May 2024.