Downing Street said Britain would also dispatch a new anti-ship missile systems to support the country against the Russian invasion.
The announcement followed talks between the two leaders in the Ukrainian capital, where Mr Johnson made a surprise visit on Saturday.
It is understood to be the first time the pair have met in person since the Russian invasion was launched on 24 February.
Downing Street said on Saturday the prime minister was in Kyiv to “demonstrate the UK’s steadfast solidarity with Ukraine” and to discuss military and economic support with Mr Zelensky.
“He reiterated that the UK will do everything in its power to support Ukraine’s brave fight against Russia’s brutal and unprovoked invasion and ensure its long term security and prosperity,” it said.
No 10 said this would include new military assistance of 120 armoured vehicles and new anti-ship missile systems to support Ukraine through a “crucial phase” of the war.
It came a day after the UK announced a further £100m for military assistance to Ukraine, including anti-tank and anti-aircraft missiles.
On Saturday, Mr Zelensky said Ukraine was bracing for a tough battle in the East, where Russia has been gathering its troops.
Ukrainian authorities have also advised civilians still in this area – which has become a focus of military action since Russia withdrew from areas around the capital – to flee.
Following his meeting with Mr Zelensky on Saturday, Mr Johnson said Ukraine had achieved “the greatest feat of arms of the 21st century” by pushing back Russian forces from the gates of Kyiv.
“It is because of President Zelensky’s resolute leadership and the invincible heroism and courage of the Ukrainian people that Putin’s monstrous aims are being thwarted,” he said.
“I made clear today that the United Kingdom stands unwaveringly with them in this ongoing fight, and we are in it for the long run.”
The Independent has a proud history of campaigning for the rights of the most vulnerable, and we first ran our Refugees Welcome campaign during the war in Syria in 2015. Now, as we renew our campaign and launch this petition in the wake of the unfolding Ukrainian crisis, we are calling on the government to go further and faster to ensure help is delivered.