The 28-year-old passed away in Rosenheim, Bavaria, on Thursday, leaving behind his wife Jenny and their two young children.
“I am deeply saddened by the death of Robert Müller, he has left us far too early,” said Uwe Krupp, head coach of Germany's national hockey team. “Through this death, we have lost not just a terrific team member, but a great person.”
In August 2008, Müller had surgery in an effort to remove the tumour and was back on the ice for the Cologne Sharks three months later. But in December of that year, his health worsened and he left the ice for the last time.
The Germany star was chosen by the Washington Capitals in the 2001 NHL Entry Draft, although he never played for the team, and he later won the German Championship title with both the Mannheim Eagles and Krefeld Penguins.
“I am of course shocked,” said Markus Kuhl, team manager of Mannheim Eagles. “I knew him since we were little... He was 100 percent a sportsman.”
In 2001, Müller was recruited for the national German hockey team, making two Olympic appearances in 2002 and 2006.
His most recent team, the Cologne Sharks, have retired his number 80 jersey in his memory.