The Poland U21 international scored 11 goals in 32 games across all competitions last season and becomes the latest arrival at Sampdoria, as reported by the club’s official website.
Kownacki, 20, moves for a reported fee of £3.4 million and will be eager to make a major impact in his first season at the club.
A graduate of Lech Poznan’s academy, the tall attacker broke into the senior squad in the 2013/14 campaign and scored 27 goals in 117 senior games. He won the league title in 2015 and also lifted the Polish SuperCup on two occasions.
Sampdoria finished in tenth place in Serie A last season and have made two other major signings so far this summer, with Gianluca Caprari and Nicola Murru both joining the club.
There have, however, been some significant departures from Sampdoria, with Luis Muriel, Bruno Fernandes and Milan Skriniar all moving on to pastures new.