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.
Last week, the US State Department urged both sides to observe the fragile ceasefire in eastern Ukraine.
"Defensive weapons, ones that would allow Ukraine to defend itself, and to take out tanks for example, would actually to help" to stop Russia threatening Ukraine, Mr Volker 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.
He said success in establishing peace in eastern Ukraine would require what he called a new strategic dialogue with Russia.
Mr Volker is a former US permanent representative to Nato and was appointed to his new role earlier this month.
The UN says more than 10,000 people have died since the eastern Ukraine conflict erupted in April 2014, soon after Russia annexed Ukraine's Crimea peninsula. The fighting has displaced more than 1.6 million people.
A ceasefire was agreed in Minsk in February 2015, but its terms are far from being fulfilled.
Last week the Ukrainian military said five of its soldiers died in heavy shelling just north of rebel-held Donetsk.
Three Ukrainian troops also died earlier when a mine blew up, the military said.
The US Department of State called it "the deadliest one-day period in 2017" in the eastern Ukraine conflict.
In a video statement, the department blamed the "Russian-led" rebels for the flare-up.
The new US special representative for Ukraine says Washington is actively reviewing whether to send weapons to help those fighting against Russian-backed rebels.