Matias, released by the Owls at the end of last season, has returned to his native Portugal after a four-year stint in England.
The 30-year-old was hampered by injury problems for long periods of his Wednesday spell, limiting him to just 71 appearances in all competitions.
The former Nacional player netted seven goals in 34 outings during the 2018/19 campaign but was one of six players who left Hillsborough last summer.
Belenenses are currently third-from-bottom in Portugal’s top-flight after winning just one of their opening seven league matches.
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