Sappinen came through the youth system at Flora and made his debut in the Meistriliiga on 2 March 2013, in a 4–1 home win over Tallinna Kalev. He won his first Meistriliiga title in the 2015 season. Sappinen's performances in 2015 also saw him voted Estonian Young Footballer of the Year.[3] He was Flora's top goalscorer in the league for three consecutive seasons from 2015 to 2017, and joint top scorer in the Meistriliiga in 2017, alongside Albert Prosa. Sappinen won his second Meistriliiga title in the 2017 season, and was named Meistriliiga Player of the Year.[4]
Loan to Beerschot Wilrijk
On 31 January 2018, Sappinen joined Belgian club Beerschot Wilrijk on loan until the end of the season.[5] He made his debut for the club on 9 February 2018, in a 0–0 home draw against Westerlo.[6]
Loan to Den Bosch
On 13 August 2018, Sappinen joined Dutch club Den Bosch on loan until the end of the season.[7] He made his debut in the Eerste Divisie on 17 August 2018, and scored the winning goal in a 2–1 away victory over Volendam.[8]
Loan to Domžale
On 14 August 2019, Sappinen penned a three-year loan deal with Slovenian club Domžale.[9]
Return to Flora
On 6 July 2021, Sappinen scored both goals in Flora's win over Hibernians in the first leg of their UEFA Champions League first qualifying round match.[10] He would score again in the second leg a week later as Flora won 5–0 on aggregate and progressed to the second qualifying round for just the third time in club history.[11]
Poland
On 22 December 2021, Sappinen signed a three-and-a-half-year contract with Polish Ekstraklasa club Piast Gliwice, effective from 1 February 2022.[12]
On 24 January 2023, Sappinen joined Stal Mielec on loan until the end of the season with an option to buy.[13]
Hapoel Jerusalem
In July 2023, Sappinen signed a one-year contract with Hapoel Jerusalem with an option for a two-year extension.[14] Already on 19 August in a derby game against Beitar in the League Cup, Sappinen got a serious injury that meant his season was over.[15] In September they decided to end the contract on mutual consent.[16]
Sappinen made his senior international debut for Estonia on 11 November 2015, in a 3–0 home victory over Georgia in a friendly.[17] He scored his first international goal on 19 November 2016, in a 1–1 away draw against Saint Kitts and Nevis in a friendly.[18]