Valery Heletey did not give details of the weapons being delivered or name the countries involved.
A similar statement earlier was denied by five NATO members, including the US.
Ukraine's prime minister says NATO is the "only vehicle" to protect the country from Russia, which he and the West accuse of intervening in the east.
Moscow has denied sending troops to help pro-Russian rebels who have been engaged in heavy fighting with government forces in the Donetsk and Luhansk regions since April.
At least some 3,000 people have been killed in five months of conflict.
A fragile ceasefire has been in place since 5 September, despite recent outbreaks of violence around key flashpoints such as Donetsk airport.
No lethal assistance
Mr Heletey told reporters on Sunday that he had discussed weapons deliveries in bilateral meetings with NATO defence ministers during a NATO summit in Wales earlier this month.
"I have no right to disclose any specific country we reached that agreement with. But the fact is that those weapons are already on the way to us - that's absolutely true, I can officially tell you," he said in remarks quoted by Ukraine's Unian news agency.
While the issue of restoring Ukraine's nuclear arsenal was not on the agenda, Mr Heletey said it would not be ruled out in future.
NATO officials say they have no plans to send lethal assistance to non-NATO member Ukraine, but that members states may do so.
But defence officials in Italy, Poland and Norway have all denied plans to provide any weapons.
On Saturday, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of wanting to "eliminate his country," saying it was the "key aggressor" in the conflict.
NATO says Russia still has about 1,000 heavily armed troops in eastern Ukraine and about 20,000 more near the border.
But Russia denies sending direct military help to the rebels, insisting that any Russian soldiers there are "volunteers".
NATO has announced the formation of a new "spearhead" force numbering several thousand troops, which can be deployed anywhere in the world in a matter of days.
It followed growing concern from NATO member countries bordering Russia over its involvement in Ukraine.
Source: BBC News Europe