Alfred Schreuder's side spent a reported €14 million to bring the Mali international as they look to bolster their midfield ahead of the 2019-20 season.
The defensive midfielder signed a five-year deal which will keep him at Hoffenheim until June 2024.
During his three-year stint in Austria, Samassekou was a crucial player and helped Salzburg win the Austrian Bundesliga consecutively for three seasons, and the Austrian Cup on two occasions.
Samassekou joins Algeria's Ishak Belfodil at the Rhein-Neckar-Arena and he is in contention to make his debut when Hoffenheim begin their 2019-20 Bundesliga campaign against Eintracht Frankfurt on Sunday.
"We have often emphasised in recent weeks that we will be active again in the transfer market, should we have an opportunity that convinces us in every way,” the club’s director of football, Alexander Rosen said.
"Diadie is a player who belongs to one hundred percent in this category.
The midfielder is extremely exciting and promising, he has tremendous potential and, despite his age, can look back on a wealth of experience in international missions, and to be able to bind such a high-caliber player to this framework with such power and intelligence, it is a great pleasure and also a dose of pride."