Halman was reportedly killed at his home in Rotterdam. Dutch news agency ANP said Jason Halman, Greg Halman's 22-year-old brother, had been arrested in the case.
Halman appeared in 44 games for the Mariners over the last two seasons. He hit .230 in 35 games in 2011 with two home runs and six runs batted in.
"Greg was a part of our organization since he was 16, and we saw him grow into a passionate young man and talented baseball player. He had an infectious smile that would greet you in the clubhouse, and he was a tremendous teammate. Our thoughts and prayers go out to Greg's family."
Halman was born Aug. 26, 1987, in Haarlem, Netherlands, and broke into the major baseball league in his native country in 2003 as a 16-year-old. His father Eddy Halman and brother Jason, like Greg Halman, played on the Dutch national baseball team.
Greg Halman was signed at Seattle as a free agent in 2004 and made his first U.S. major-league appearance in September 2010.