The 29-year-old has signed a two-year contract at the Stadium of Light and becomes the 10th signing under new boss David Moyes.
Mika, whose full name is Michael Simoes Dominguez, is set to challenge Jordan Pickford and Vito Mannone for the regular number one position.
Pickford has been the only fit senior goalkeeper at the club since Mannone tore elbow ligaments last month, ruling him out until at least November.
Mika has spent the last two seasons with Boavista and represented his country at under-21 level
The deal has been ratified by FIFA despite coming one week after the end of the transfer window, due to what is described as a "technical issue" on the selling club's part
Source : PA
Source: PA