Nadir arrives on a free transfer after his contract at Olympique Marseille expired at the end of the season, meaning no fee is due for the Morocco international.
He made 28 appearances in all competitions for the French side last term, contributing one goal and five assists as Marseille competed for European qualification, and has signed a long-term contract at the Volksparkstadion having beaten VfB Stuttgart and Eintracht Frankfurt to his signature in the race for the versatile midfielder.
HSV sporting board member Kathleen Kruger explained what the left-footed midfielder will add to Merlin Polzin's squad.
"Bilal is a very versatile and adaptable player who can take on various roles in midfield. His signing gives us additional flexibility in this area of the team and further strengthens our squad both in depth and in quality," Kruger said.
Nadir himself expressed his delight at completing the move to Hamburg.
"I'm very happy to now be a player of HSV. I can't wait to meet the HSV fans and play for Hamburg," Nadir said.
The Nice-born midfielder came through the ranks at OGC Nice before making the move to Marseille in 2021, going on to make 53 appearances for the club's first team across various spells in the south of France.
His versatility in midfield is expected to give HSV greater options as they continue preparations for the new Bundesliga season, with the club having already been active in the French market this summer following the earlier arrival of Adeline.
HSV's business is not expected to stop there, with the club also continuing to explore other targets as they look to build a squad capable of competing in the top half of the table following their promotion back to the Bundesliga last year.
Nadir is set to join up with his new teammates in the coming days as Hamburg continue their pre-season preparations ahead of the new Bundesliga campaign, with the club still hopeful of adding further reinforcements in defence and attack before the transfer window closes at the start of September.