Ukraine can still win the war if given the right support, he said.
He also announced the US would allocate an extra $713m (£559m) of military aid to Ukraine and other European nations.
At a news conference in Poland after the visit, Mr Austin told reporters the US wants to see "Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine".
Mr Austin, a retired four-star general, was speaking after meeting Ukraine's President Volodymyr Zelensky in Kyiv. Also involving Secretary of State Antony Blinken, the visit marked the highest-level trip to Ukraine by US officials since the invasion began over two months ago.
The meeting between the US and Ukrainian parties, which ran for over three hours, comes as Russia escalates its military campaign in the south and east of the country.
