
The German midfielder has put pen to paper on a three-year deal and arrives after leaving Bundesliga side Paderborn.
He scored 17 goals in all competitions in the Bundesliga 2 last season and helped Paderborn win promotion to the German top flight for the first time since 2019/20.
Werder Bremen had been linked with Bilbija who is an attacking midfielder and he has already linked up with his new Derby team-mates in Austria.
He could potentially feature in the friendly against German side Freiburg on Friday in what will be the Rams' latest pre-season assignment.
Bilbija was a product of the Berlin youth academies, including Tennis Borussia Berlin and FC Hertha 03 Zehlendorf.
He began his professional career with FC Ingolstadt 04 in 2019 and was a key figure in Ingolstadt's promotion back to the Bundesliga 2 in 2021.
In the summer of 2022, he transferred to Hamburger SV on a four-year deal. After a single season there, he moved to SC Paderborn 07 in 2023, where he remained a versatile offensive contributor in the second tier before becoming a free agent in July 2026.
He is Derby's third signing of the summer and follows the arrival of Bobby Clark who was a £6m capture from RB Salzburg while Charlie Taylor also put pen to paper on a free transfer after leaving Southampton.