Rotterdam Police spokeswoman Patricia Wessels says police were called to a home in the Dutch port city early Monday morning and found Halman bleeding from a stab wound.
The officers attempted unsuccessfully to resuscitate the 24-year-old.
Wessels says the officers arrested Halman's 22-year-old brother. She declined to give his name, in line with Dutch privacy rules.
She said the brother was being questioned by police.
The Dutch-born Halman appeared in 44 games over the last two seasons for Seattle. He hit .207 with two homers and six RBIs.