He delivered a defiant speech at an event marking the centenary of the ruling Communist Party on Thursday.
Mr Xi also said Beijing would not allow "sanctimonious preaching", in remarks widely seen as directed at the US.
It comes as China faces criticism over alleged human rights abuses and its crackdown in Hong Kong.
Relations between the US and China have worsened in recent times over trade, espionage and the pandemic.
The issue of Taiwan is also a major source of tension. While democratic Taiwan sees itself as a sovereign state, Beijing views the island as a breakaway province.
The US, under its own laws, is required to provide Taiwan with the means to defend itself should Beijing use force to take the island back.
On Thursday Mr Xi said China maintains an "unshakeable commitment" to unification with Taiwan.
"No one should underestimate the resolve, the will and ability of the Chinese people to defend their national sovereignty and territorial integrity," he said.