Born in Fukuoka, Japan, Tomiyasu was first scouted by the Sanchiku Kickers’ general manager Kanji Tsuji, who was impressed with his performance, especially his speed.

[4] Following this, Tsuji convinced his parents to let him play football, which led Tomiyasu to be enrolled at Sanchiku Elementary School.

[5] He impressed at the trial so FC Barcelona offered him a place on the club's youth academy, but it was difficult for him to relocate to Spain and the move was cancelled.

[15] he made his first appearance of the season on 20 April 2016, as a starter in the defensive midfield position, in a 0–0 draw against Vegalta Sendai in the J League Cup.

[17] Two months later, on 13 July 2016, he made his league debut for the club, starting in the defensive midfield position in a 2–1 win against FC Tokyo.

It was announced on 8 January 2018 that Tomiyasu would be moving abroad for the first time in his career by joining Belgian First Division A side Sint-Truiden.

"[34] After spending months at training and recovering from an ankle injury, Tomiyasu made his Sint-Truiden debut for the club, coming on as a late substitute, in a 2–1 win against Royal Antwerp on 12 May 2018.

[39] After spending January on international duty with Japan, Tomiyasu returned to Sint-Truidense on 9 February 2019, starting and playing the full 90 minutes in a 2–1 win against Waasland-Beveren.

[47] A week later, he made his league debut, playing the full 90 minutes of a 1–1 draw against Hellas Verona,[48] and was named Man of the Match for his performance.

[52] Since joining the club, Tomiyasu quickly established himself in the starting line-up playing at right back, and his performances impressed the Italian media and supporters alike.

[75] Upon joining the club, both manager Mikel Arteta and technical director Edu commented about the player, describing him as an "important member of the squad" at Arsenal.

[76] He became the fourth Japanese player to sign for the club after Junichi Inamoto and Ryo Miyaichi; others such as Takuma Asano never managed to feature in a competitive senior fixture for Arsenal.

[78] After the match, Tomiyasu’s debut performance at the club was praised by manager Arteta and local newspaper, Football London.

[82] Following a 2–0 win against Leicester City on 30 October, manager Arteta praised his impact to the team and his settling to the country.

[96] In a match against Manchester United on 22 January 2023, Tomiyasu came on as a second–half substitute and played a role when he set up a goal for Bukayo Saka to take a 2–1 lead, as the club won 3–1.

[97] Three week later on 15 February 2023 against Manchester City, Tomiyasu started the match and was at fault when he made a "poor backpass allowed Kevin de Bruyne to loft a finish over Aaron Ramsdale after 24 minutes", as Arsenal loss 3–1.

[99] However, he sustained a knee injury and was substituted in the 9th minute during Arsenal’s loss against Sporting CP in the second leg of the UEFA Europa League last 16.

[103] However on 21 August 2023, Tomiyasu made his first start of the 2023–24 season, in Arsenal's 1–0 win at Crystal Palace, where he was controversially sent off in the 67th minute for two bookable offences.

[105] On 27 September, Tomiyasu was finally used as a centre back for Arsenal in the 2023–24 EFL Cup third round match against Brentford and contributed to a 1–0 clean sheet victory.

[107] Three weeks later on 28 October, Tomiyasu scored his first goal for the club in the 96th minute of stoppage time in a 5–0 victory over Sheffield United.

[109] On 2 November, Tomiyasu was selected as the team's monthly MVP for October by fan voting, making it his second award since September 2021.

[142] Tomiyasu made his debut in the tournament against France Olympic team on 28 July 2021 and started the whole game to help Japan win 4–0.

[150] He helped Samurai Blue keep a clean sheet by beating Oman 1–0 win to advance to the knockout stage.

[153] In the AFC Asian Cup final against Qatar, he started and captained Japan, as the Samurai Blue loss 3–1, finishing as runner–up in the process.

[154] Following the conclusion to the AFC Asian Cup tournament, Tomiyasu was called up to the Japan squad ahead of the Copa América in Brazil.

[165] Tomiyasu made his first start in the centre–back position, forming a partnership with Yoshida, against Croatia in the last-16 of the World Cup, as the match eventually ended 1–1 with the Samurai Blue crashing out after their defeat on penalties.

[172] His performance at Sint-Truiden was so impressive that Voetbal Nieuws named him "The Best Eleven of Foreign Players Who Have Left from the Jupiler League".

[174] Bologna's technical coach Emilio De Leo said about Tomiyasu's performances, saying: "He has the ability to read and manipulate time and space in a modern way.

"[175] The club manager Marco Di Vaio agreed, saying: "Tomiyasu immediately impressed us because he is very good with his feet and knows how to adapt to all defense roles.

"[179] Reflecting on his first season at Arsenal, Football London said about Tomiyasu: "Defensively immaculate and technically excellent he instantly transformed the right back position from one of weakness into one of strength.