Predominantly a central defender, 25-year-old Ayhan can also play as a holding midfielder and right-back.
Dusseldorf’s relegation to the 2. Bundesliga means the Turkey international is on his way out after four seasons.
Atalanta had Ayhan in their sights last year, but another of Serie A’s great entertainers, Sassuolo, have secured him.
Ayhan, who was born in Germany and rose through the Schalke youth system, made 31 appearances for Dusseldorf this season.
Since 2016, he has won 28 caps for Turkey after choosing to represent them over Germany.