The head of the Polish Football Association (PZPN), Cezary Kulesza, told a news conference in Warsaw that Santos, who previously coached Greece and Portugal, was "the best manager available" to take over from Michniewicz, whose role as Poland boss ended on December 31.
The national soccer governing body in December decided against renewing Michniewicz's contract following what many described as Poland's uninspiring performance at last year's World Cup in Qatar.
Santos, 68, was in charge of the Portuguese national team from 2014 to 2022, helping it reach the quarterfinals of the World Cup in Qatar.
He previously guided Portugal to the 2016 European championship title and the 2019 Nations League trophy.
Santos was also in charge of the Greece national team from 2010 to 2014, leading them to the 2012 European championships and the 2014 World Cup in Brazil.
He takes over as Poland boss as the team prepares for its Euro 2024 qualifying matches.
The Polish team has been drawn to play the Czech Republic, Albania, the Faroe Islands and Moldova in qualifiers for the Euro 2024 championships in Germany.
The Poles are due to start their Euro 2024 qualifying campaign with an away game against the Czech Republic on March 24.
Poland reached the last 16 of last year's World Cup in Qatar, where they were eliminated by eventual runners-up France.
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Source: IAR, PAP, TVP Info