Martinez is one of several strikers to have left West Ham United since the end of last season.
The Spanish striker spent the bulk of his West Ham career out on loan and finally saw his contract expire at the end of June.
He is now due to spend the next three seasons in Portuguese football, with the newly-promoted top-flight side, FC Famalicão.
Martinez told the club's official website that he is "delighted to be invited to be part of an ambitious project" and expressed a desire to "help FC Famalicão achieve many victories".
The former West Ham player is the third to join Famalicao from an English club this summer after Ruben Lameiras (Plymouth Argyle) and Josh Tymon (Stoke City).
Martinez moved to the London Stadium from Valencia three years ago and cost a reported fee of £2.7 million.
But the 22-year-old only appeared three times for West Ham's first team and provided just one assist - the winner in an FA Cup replay against Shrewsbury Town 18 months ago.
West Ham have experienced a clearout of strikers since the end of last season, with Martinez joining Andy Carroll, Lucas Perez and Marko Arnautovic in leaving east London, making way for the signing of Sebastien Haller.
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