He is Joseph Stone, 36, according to the Organization for Security and Cooperation in Europe's Special Monitoring Mission. There was no further detail released about Stone.
It is the first time a member of an SMM patrol has been killed while on duty.
Stone died and two other monitors were injured on Sunday after an explosion damaged a mission vehicle near Pryshyb in eastern Ukraine. A mine might have triggered the blast.
The injured monitors, a German woman and a Czech man, were transported to Kramatorsk on Monday and will be taken to Kiev where they will receive "appropriate care," said the mission's spokeswoman Alexandra Taylor.
The SMM is an unarmed civilian mission tasked with working to help normalize and stabilize the situation in eastern Ukraine.
Fighting continues between pro-Russian separatists and Ukrainian armed forces in eastern Ukraine, according to OSCE Special Monitoring Mission Chief Monitor Ertugrul Apakan said Monday.
"I reiterate my call for sustainable cease-fire, withdrawal of weapons, full demining and real commitment to peace. And I ask that those responsible for placing mines are held accountable," Apakan said.