The 20-year-old arrives in the Bundesliga after agreeing a four-and-a-half year deal with Frank Kramer's side, who are currently in 14th place in the standings.
"The last few hours have been very exciting," Bello said in a statement on Bielefeld's website. "My transfer to Arminia is a dream come true for me, because I've always wanted to play football in Europe. I'm happy and grateful that Arminia are giving me this opportunity. I can develop as a player and as a person here. I can hardly wait to get started and I'm looking forward to playing in the Bundesliga."
Bello was born in Nigeria but moved to the US as a baby. He joined Atlanta's youth academy at the age of 14 and swiftly progressed through the ranks, eventually signing an MLS homegrown professional contract with the club in June 2017 and making his top-flight debut in September 2018 when he was just 18.
The left-footer soon went on to establish himself as a first-team regular and also earned the first of six senior USMNT caps so far in a 7-0 win over Trinidad and Tobago in January 2021.
"In George, we're getting a fast and dynamic left-back who also loves to get forward," said head coach Kramer. "He's a good guy and he's got that typical American enthusiasm and wants to make a difference."