The 36-year-old, who captains Poland's national football team and plays for Barcelona at club level, picked up the injury during his club's 4-3 win over Celta Vigo in the Spanish La Liga on Saturday.
He was substituted in the 78th minute and later diagnosed with a tear in the left semitendinosus muscle, Polish state news agency PAP reported.
"Tests carried out on Sunday have confirmed that first-team player Robert Lewandowski has a left semitendinosus injury," the club said in a statement.
"His return will be determined by his progress," it added.
The Pole is expected to be sidelined for around three weeks, missing several high-profile fixtures, including the Copa del Rey final against Real Madrid next Saturday and at least the first leg of the Champions League semifinal against Inter Milan, the PAP news agency reported.
Saturday marked Lewandowski's 100th La Liga appearance. Though he did not find the net, he "is enjoying his most prolific season in a Barça jersey," the club said on its website.
The Polish marksman has scored 40 goals across all competitions so far this season: 25 in La Liga, 11 in the Champions League, three in the Copa del Rey, and one in the Spanish Super Cup.
His 25 league goals place him at the top of the Pichichi trophy standings, ahead of Real Madrid's Kylian Mbappé, Osasuna's Ante Budimir, and teammate Raphinha.
"Time will tell just how far the FC Barcelona striker—now a La Liga centurion—can push his goal tally," the club added.
(gs)
Source: PAP, fcbarcelona.com, polskieradio24.pl