Rotterdam Police spokeswoman Patricia Wessels said police were called to a home in the port city in the early hours of the morning and found the 24-year-old Dutch player bleeding from a stab wound.
The officers and ambulance paramedics were unable to resuscitate Halman.
Wessels said the officers arrested Halman’s 22-year-old brother. She declined to give his name, in line with Dutch privacy rules.
No charges have been filed.
Halman hit .230 in 35 games and made starts at all three outfield positions for the Mariners in 2011 before being optioned to Triple-A Tacoma.
Mariners Chairman Howard Lincoln, President Chuck Armstrong and general manager Jack Zduriencik paid tribute to Halman on behalf of the club.
“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,” they said in a statement. “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.”
Former major leaguer Robert Eenhorn, the technical director of the Dutch baseball association, said he was devastated by the news.
“The only thing I can say right now is we are deeply shocked,” he said.