The Warsaw club finished the 2019/2020 campaign three points ahead of Lech Poznań and eight points ahead of defending champions Piast Gliwice to regain their title.
In the final round of matches on Sunday, Legia, managed by former Serbian midfielder Aleksandar Vuković, lost 1-2 to Pogoń Szczecin at home, while Lech posted a comfortable 4-0 home win against Jagiellonia Białystok.
Piast drew 1-1 with Cracovia Kraków in front of a home crowd in the southern city of Gliwice.
Earlier this month, Legia beat Cracovia 2-0 in a home fixture to secure its 14th Polish Ekstraklasa championship, with two games to spare in a season extended because of the coronavirus pandemic.
Polish football stadiums reopened to spectators at 25 percent capacity last month amid the COVID-19 pandemic.
Last year, Legia Warsaw finished runner-up in the Polish premier league, while Piast Gliwice, a small-budget side managed by ex-Poland boss Waldemar Fornalik, captured their maiden title.
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Source: TVP, Canal+, IAR