
Castelan, 24, comes to Hamburg from Feyenoord Rotterdam and reportedly costs a transfer fee of 2.5 million euros (3.4 million dollars). He has already passed the medical check.
The 10-times capped Castelan is the fourth Dutch player in Hamburg, joining Nigel de Jong, Joris Mathijsen and team captain Rafael van der Vaart. Hamburg also have a Dutch coach in Huub Stevens.
Hamburg have made several hirings after only narrowly avoiding relegation in the past season, most notably Egyptian forward Mohamed Zidan for 5 million euros from relegated Mainz.
Hamburg are also close to finalising the hiring of German under-19 international Kevin Boateng from Hertha Berlin, who is said to be worth 1 million euros.
The German Bundesliga season starts on August 10.
DPA