He was linked with moves to numerous major clubs in January 2020; on deadline day, Birmingham were reported to have turned down a £20 million bid from Manchester United. Bellingham continued as a first-team regular, and by the time the season was suspended because of the COVID-19 pandemic, he had made 32 league appearances. He remained an integral part of the team once the season resumed behind closed doors, and set up a late equaliser for Lukas Jutkiewicz against Charlton Athletic that made Birmingham's league position less precarious with two matches still to play. He finished the season with four goals from 44 appearances in all competitions, 41 in the league, as Birmingham avoided relegation despite losing the last match of the season. In appreciation of what Bellingham achieved in such a short time with the first team, the club announced that they would retire his number 22 shirt, "to remember one of our own and to inspire others." At the EFL Awards, he was named both Championship Apprentice of the Year and EFL Young Player of the Season.
Bellingham made his debut on 14 September 2020 in the starting eleven for the first fixture of Borussia Dortmund's 2020–21 season, against third-tier MSV Duisburg in the DFB-Pokal, at the age of 17 years, 77 days. After half an hour, he scored the second goal in a 5–0 win, and became not only the club's youngest goalcorer in the DFB-Pokal, taking six days off Giovanni Reyna's record, but also their youngest scorer in any competitive match, breaking Nuri Şahin's record by five days. Five days later, he marked his Bundesliga debut with the assist for Reyna's opening goal in a 3–0 win over Borussia Mönchengladbach, and was named as Bundesliga Rookie of the Month for September. When Bellingham faced Lazio in the group stage on 20 October, aged 17 years and 113 days, he became the youngest Englishman to start a Champions League match, breaking the record previously set by Phil Foden.
Bellingham received his first call-up to the under-21 squad for European Championship qualifiers against Kosovo and Austria in September 2020. He became the youngest player to appear for England U21 when he came on to replace Tom Davies after 62 minutes of the match against Kosovo on 4 September with England 3–0 ahead, and scored after 85 minutes to complete the 6–0 victory.
On 10 November 2020, Bellingham was called up to the England senior team for the first time following the withdrawals of James Ward-Prowse and Trent Alexander-Arnold through injury. He made his debut in a friendly against the Republic of Ireland at Wembley on 12 November, replacing Mason Mount after 73 minutes of a 3–0 win. At 17 years, 136 days, he became England's third-youngest full international; only Theo Walcott and Wayne Rooney had appeared at a younger age.