Derek Chauvin, 45, was filmed kneeling on Mr Floyd’s neck for more than nine minutes during his arrest last May.
The widely watched footage sparked worldwide protests against racism and excessive use of force by police.
Chauvin was found guilty on three charges: second-degree murder, third-degree murder and manslaughter.
His bail was immediately revoked and he was placed in custody. Sentencing is likely to happen in two months, and Chauvin could spend decades in jail.
In Minnesota, second-degree murder carries a maximum sentence of 40 years in prison. Third-degree murder is punishable by up to 25 years in prison. Second-degree manslaughter is punishable by up to 10 years in prison.
Chauvin is expected to appeal against the verdict.
Floyd’s death on Memorial Day 2020 sparked protests in Minneapolis, across the United States and around the world. It prompted calls for police reform and soul-searching on issues of racial injustice.
Floyd was a 46-year-old Black man from Houston who had moved to Minnesota three years earlier. He was a father and brother who idolized his mother, loved making music and had been a star athlete as a young man.
Floyd died after Chauvin pressed his knee on Floyd’s neck for 9 minutes and 29 seconds as Floyd lay facedown, hands cuffed behind his back.