Lech secured their eighth Polish Ekstraklasa championship after beating local rival Warta Poznań 2-1 on Saturday.
Lech's closest rival, Raków Częstochowa, lost 0-1 to Zagłębie Lubin that same day.
Lech Poznań manager Maciej Skorża. Photo: PAP/Jakub Kaczmarczyk
Meanwhile, top-flight powerhouse Wisła Kraków stand to be relegated from the Polish premier league.
The Kraków team, managed by former Poland boss Jerzy Brzęczek and co-owned by national team ex-captain Jakub Błaszczykowski, lost their chance of keeping their Ekstraklasa hopes alive after suffering a 2-4 defeat to Radomiak Radom during the penultimate round of matches.
Lech and Raków will represent Poland in European competitions, which are due to begin later this year. Joining them will be Pogoń Szczecin, who are set to finish third in the Polish premier league, and fourth-place finishers Lechia Gdańsk.
The Polish premier league season ends next weekend.
Earlier this month Raków beat Lech 3-1 to win the Polish Cup.
Raków and Lech will face off against each other again for the national Super Cup title later this year in a curtain-raiser to the next football season.
Source: TVP, PAP