Kurt Volker told the BBC that arming Ukrainian government forces could change Moscow's approach.
He said he did not think the move would be provocative.
Russia warned that anything that heightened tension could jeopardise a solution to the conflict.
Mr Volker, a former US permanent representative to Nato, was given the role in Kiev earlier this month.
"Defensive weapons, ones that would allow Ukraine to defend itself, and to take out tanks for example, would actually help" stop Russia threatening Ukraine, he said in a BBC interview.
"I'm not again predicting where we go on this. That's a matter for further discussion and decision. But I think that argument that it would be provocative to Russia or emboldening of Ukraine is just getting it backwards," he added.