The 25-year-old arrives at the Stadium of Light following approval from FIFA after the Portuguese club were unable to complete their requirements before the Aug. 31 transfer deadline due to technical issues.
Sunderland's first-choice goalkeeper Vito Mannone has been ruled out for around three months due to an elbow injury with Jordan Pickford deputising.
Mika, 25, made 26 appearances for Boavista as they finished 14th in the Portuguese league table last season.
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