The incident took place in the northern city of Ovruchi, where the perpetrator was being examined by border guards.
“A 23-year-old resident of Kharkiv, who was in possession of explosives, was killed. Three women, aged 29, 58 and 82, (local) residents of the Korosten district, were also fatally injured,” police said in a statement.
Twelve others were injured and taken to hospital for treatment.
According to Ukrainian news site Ukrinform, preliminary information suggests that the man who fired the grenade was wanted by authorities.
Police, pyrotechnicians and forensic experts from regional headquarters are investigating, and the site of the explosion remains sealed off as authorities try to determine the motive for the attack.
Although it is 40 km from Belarus, Ovruchi station is a border control area.
MNA/