Article 5 of Nato's treaty says that if an ally is a victim of an attack, every other member will "consider the act of violence as an armed attack against all members and will take the actions it deems necessary to assist the ally attacked".
"We have already made our way to Nato," Zelensky said. "We have already proven compatibility with the Alliance's standards.
"They are real for Ukraine - real on the battlefield and in all aspects of our interaction. We trust each other, we help each other, and we protect each other. That's what the Alliance is. De facto.
"Today, Ukraine is applying to make it de jure. In a process that is consistent with our value in protecting our entire community. In an expedited manner.
"We are taking our decisive step by signing Ukraine's application for accelerated accession to Nato."
Putin's annexation of the four regions follows a series of sham referendums that have not been recognised internationally.
The total area claimed covers 90,000sq km, around the same size as Hungary.
Zelensky vowed to reunite Ukraine.
"The entire territory of our country will be liberated from this enemy - the enemy not only of Ukraine, but also of life itself, humanity, law and truth," he said.
"Russia already knows this. It feels our power. It sees that it is here, in Ukraine, that we prove the strength of our values."
Earlier, at the Kremlin, Putin made a series of wild claims during a speech beamed across the nation.
"People have made their choice - and that choice is beyond any doubt," Putin said.
He vowed to "defend our land with all means" - raising fears of a nuclear escalation in the war with Ukraine.
"People living in Luhansk, Donetsk, Kherson region and Zaporizhzhia region are becoming our citizens forever," he added.
Putin claimed "millions of people" had "opted" to become Russian, despite voting in phoney referendums while being held at gunpoint.
He also claimed people in the Donbas region had been "victims of inhumane terrorist attacks conducted by the Kyiv regime".
Putin also made a thinly veiled nuclear threat, saying the US had started a "precedent" by using atomic bombs against Japan at the end of World War II.
He added that he did not want to rebuild the Soviet Union, but said people in the occupied territories shared a "common history" with Russia.
"Despite all the difficulties, they have carried through this love for Russia, and this feeling cannot be exterminated by anyone," Putin said.
"The USSR is no more. We can't bring the past back. And Russia doesn't need it any more. We are not striving towards that," Putin said, after earlier claiming the last Soviet leaders had "destroyed our great country".
The Russian tyrant called on Kyiv to return to the negotiating table, but said they must "respect the expression of the will of the people".
"We will protect our land using all our forces and we will do everything to ensure people's securities," he added.
Putin also used the speech, which was projected onto a big screen in Red Square, to slam Western powers.
The West is looking new opportunities to hit us and they always dreamt about breaking our state into smaller states who will be fighting against each other," Putin said.
"They don't want to see us a free society. They want to see us as a crowd of slaves.
"They don't need Russia. We need Russia.
"They have been colonisers and they remain colonisers, they discriminate and they distinguish between the first class of nations and second class nations."
The annexation was immediately met with condemnation.
The US immediately imposed new sanctions on hundreds of business leaders and politicians in Russia.
President Joe Biden branded Putin's annexation "fraudulent" and said the US would not recognise the regions as part of Russia.
EU leaders vowed they would "never recognise" Russia's illegal claims over Ukraine and accused the Kremlin of putting global security at risk.
"We firmly reject and unequivocally condemn the illegal annexation by Russia of Ukraine's Donetsk, Luhansk, Zaporizhzhia and Kherson regions," the 27 leaders said in a statement.
