He added that while "my expectation is that it [a Chinese invasion] will not happen, it will not be attempted", at the same time it depended on "how strong the world makes clear that that kind of action is going to result in long term disapprobation".
He was then asked directly if the US would defend Taiwan militarily if China invaded, when it has not done so in the invasion of Ukraine, and he responded: "Yes... that's the commitment we made."
"The idea that it can be taken by force... is just not appropriate. It will dislocate the entire region and be another action similar to what happened in Ukraine."
But Mr Biden also prefaced his remarks saying US policy toward Taiwan "has not changed", which a White House spokesman swiftly reiterated.
The remarks in Tokyo were the second time in recent months Mr Biden has unequivocally stated the US would defend Taiwan if China attacked, in what has been seen as a change in tone.
The US pursues a policy known as "strategic ambiguity" on the China-Taiwan issue, and previously has been vague on what it would do in such a situation.
It is constantly walking a diplomatic tightrope due to commitments to both China and Taiwan.
It has no official diplomatic ties with Taiwan, but sells arms to it as part of its Taiwan Relations Act, which states that the US must provide the island with the means to defend itself.
At the same time, it maintains formal ties with China and also diplomatically acknowledges China's position that there is only one Chinese government, which is known as the One China policy.
Mr Biden on Monday stressed that the US still supports this policy.
China's foreign ministry spokesman Wang Wenbin insisted "Taiwan is an inalienable part of China's territory" and there was no room for compromise.
"The Taiwan question and the Ukraine issue are fundamentally different. To compare those two is absurd. We once again urge the US to abide by the One China principle."
China and Taiwan: The basics
Why do China and Taiwan have poor relations? China and Taiwan were divided during a civil war in the 1940s, but Beijing insists the island will be reclaimed at some point, by force if necessary
How is Taiwan governed? The island has its own constitution, democratically elected leaders, and about 300,000 active troops in its armed forces
Who recognises Taiwan? Only a few countries recognise Taiwan. Most recognise the Chinese government in Beijing instead. The US has no official ties with Taiwan but does have a law which requires it to provide the island with the means to defend itself