Bielefeld began the season with a 1–1 draw at
St. Pauli, thanks to a 90th minute
Manuel Prietl header rescuing a point for Bielefeld. They also drew their second game, a 3–3 draw at home to
VfL Bochum, before recording their first win of the season with a 3–1 victory at home to
Erzgebirge Aue. Bielefeld picked up 10 points from their next 4 league games before, their unbeaten run was broken with a 1–0 defeat at home to
VfB Stuttgart, with a 91st minute
Hamadi Al Ghaddioui strike consigning Arminia Bielefeld to their first league defeat of the season. However, Bielefeld bounced back from this defeat with a 1–0 away victory at
VfL Osnabrück. Following a draw at home to
Hamburger SV, They won their next three league games against
Dynamo Dresden,
Holstein Kiel and
1. FC Nürnberg to move to the top of the
2. Bundesliga. Though a 1–1 draw at home to
SV Sandhausen meant Hamburger SV returned to the top of the table, Bielefeld returned to the top of the league with two goals from
Fabian Klos and one from
Andreas Voglsammer securing a 3–1 win at
SV Darmstadt 98. Arminia Bielefeld entered the winter break top of the
2. Bundesliga on 34 points. During the
January transfer window, Bielefeld signed goalkeeper
Oscar Linnér from
AIK Fotboll and young forward
Sebastian Müller from
1. FC Köln, whilst no players left the club. Following the winter break, Bielefeld beat
VfL Bochum 2–0 at home on 28 January 2020, before drawing 0–0 at Erzgebirge Aue on 31 January 2020. Bielefeld scored 10 goals in their following two games, recording a 6–0 win at home to
Jahn Regensburg with six different goalscorers, followed by a 4–2 victory away at
Greuther Fürth. Bielefeld's next two matches were both 1–0 home victories, first against
Hannover 96 and then against
Wehen Wiesbaden. Despite going 1–0 down to 2nd placed
VfB Stuttgart, a left-footed goal from
Cebio Soukou in the 76th minute ensured Bielefeld retain their 6-point league at the top of the 2. Bundesliga. However, the
2. Bundesliga season was postponed later in March due to fears over the
COVID-19 pandemic, leaving Bielefeld's chances of promotion in doubt. After two months without football, it was announced on 7 May 2020 that German football could resume behind closed doors from 16 May 2020, meaning Bielefeld's promotion prospects were strong. Bielefeld returned to action on 17 May 2020, and took the lead against VfL Osnabrück through a 17th minute
Fabian Klos penalty before conceding a 94th-minute equaliser, though still extended their lead over second place to 7 points. Bielefeld's form was inconsistent, with the club winning one and drawing three of their following four matches. A 4–0 win at home to Dynamo Dresden on 15 June 2020 meant that Bielefeld would secure promotion to the
Bundesliga if either Hamburger SV or VfB Stuttgart failed to win in their matches on 16 and 17 June respectively. On 16 June 2020, Bielefeld's promotion to the
2020–21 Bundesliga was secured after second-placed Hamburger SV drew 1–1 at home to VfL Osnabrück, rendering it impossible for Bielefeld to finish below second place. Their final two matches saw a 3–3 draw away at
Karlsruher SC followed by a 3–0 victory at home to
1. FC Heidenheim. ==First-team squad==