The club on Tuesday announced an 18-month deal with the 34-year-old.
Boa Morte left financially-strapped Greek club Larissa after just six months in December.
Now 34, he played at Portugal's Sporting before embarking on a 14-year career in England in 1997 when he joined Arsenal. He also played at Southampton, Fulham and West Ham.
Pirates play in the upcoming African Champions League but Boa Morte's signing is too late for him to be registered for the competition.
The second half of the South African season commences on February 15.
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