former Chelsea midfielder, 31, travelled from London to the north-east yesterday after agreeing personal terms.
Two-time Premier League winner Mikel will join up with his new team-mates on Wednesday morning after spending the past two years at Tianjin TEDA in the Chinese Super League.
Roma and Wolfsburg had registered an interest but the Nigerian international wanted to return to England, where his wife and two young children are based.
The Champions League winner will be part of a Tony Pulis side looking to bring Premier League football back to the Riverside next season.
Boro currently sit fifth in the Championship table and seven points off leaders Leeds.
