Adarabioyo, 20, came close to leaving City on loan last summer, but remained part of Pep Guardiola's senior squad after Eliaquim Mangala's proposed move away from the Etihad Stadium fell through.
The centre-back made was handed a bumper new contract 12 months ago and made four appearances for City during the previous campaign, including three full outings; one in the Champions League and two in the EFL Cup.
Guardiola referred to the youngster as a "champion" and part of his title-winning squad given he trained with the senior team throughout the 2017-18 campaign.
He had received a number of offers in recent weeks but is set to move to West Brom, who are preparing for a season in the Championship.
Adarabioyo's place in Guardiola's squad - effectively the fifth-choice centre-back - could now be filled by Eric Garcia.
The 17-year-old former Barcelona defender has impressed Guardiola since arriving in Manchester last summer, and was one of the stand-out performers during City's three International Champions Cup matches in the United States.