Mr Mattis claims Russia has not kept to the Minsk ceasefire agreement to end separatist violence in the east of the war-torn nation.
In the wake of Russian troops in Ukraine, Mr Mattis said the US President is “actively reviewing” the possibility of sending lethal defensive weapons to Ukraine.
Speaking in Kiev, he said: “Despite Russia’s denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe.”
Mr Mattis’s visit is the second high-profile show of US support after the Secretary of State Rex Tillerson came to the Ukraine capital in July.
He said: “Have no doubt, the United States stands with Ukraine.”
Kiev wants the US to supply lethal defensive weapons to the Ukraine, but Mr Mattis has not promised to provide any yet.
He said: “On the defensive lethal weapons, we are actively reviewing it.
“I will go back now having seen the current situation and be able to inform the Secretary of State and the President in very specific terms what I recommend for the direction ahead.”
The Ukraine defence minister Stepan Poltorak said: “From the first day of the Russian aggression we appealed to all countries of the world to help us in the form of lethal weapons.”
Donald Trump made certain comments during his campaign trail last year, such as recognising Crimea as a part of Russia, which gave fears to the Ukraine that the US would build closer ties with Moscow.
But Mr Mattis said: “It’s not easy making a sovereign state, especially right now with the way Russia has been violating territorial integrity.”
Senator John McCain has also spoken out on the issue, calling on Trump to act.
He said: “It is long past time for the United States to provide Ukraine the defensive lethal assistance it needs to deter and defend against Russian aggression.”