At a meeting in Kyiv, Johnson told Ukraine's President Volodymyr Zelenskiy Britain would provide armoured vehicles and anti-ship missile systems, along with additional support for World Bank loans.
The support aims to ensure "Ukraine can never be bullied again, never will be blackmailed again, never will be threatened in the same way again", he said.
Johnson was the latest foreign leader to visit Kyiv after Russian forces pulled back from areas around the capital just over a week ago.
Earlier in the day, Zelenskiy met Austrian Chancellor Karl Nehammer in Kyiv, warning in a joint news conference that while the threat to the capital had receded, it was rising in the east.
"This will be a hard battle, we believe in this fight and our victory. We are ready to simultaneously fight and look for diplomatic ways to put an end to this war," Zelenskiy said.
Ukranian negotiator Mykhailo Podolyak said Zelenskiy and Russian President Vladimir Putin would not meet until after the country defeated Russia in the east, which would bolster its negotiating position.
"We are paying a very high price. But Russia must get rid of its imperial illusions," he said, according to the Interfax Ukraine news agency.
Air-raid sirens sounded in cities across eastern Ukraine, which has become the focus of Russian military action, on Saturday.
On Friday, officials said more than 50 people were killed in a missile strike on a train station in the city of Kramatorsk in the Donetsk region, where thousands had gathered to evacuate.
Russia's invasion, which began on February 24, has forced around a quarter of the population of 44 million to leave their homes, turned cities into rubble and killed or injured thousands.
The civilian casualties have triggered a wave of international condemnation, in particular over deaths in the town of Bucha, northwest of Kyiv, that until last week was occupied by Russian forces.
"We will never forget everything we saw here, this will stay with us for our whole lives," Bohdan Zubchuk, a community policeman in the town, said.
British military intelligence said Russia's retreat from the region revealed "disproportionate" targeting of civilians.
Russia has denied targeting civilians.
AAP with The Project.