Johnson will on Tuesday meet with Ukraine's President Volodymyr Zelenskiy to discuss the tens of thousands of Russian troops amassed in and near Ukraine that the West fears could be preparing to invade.
The United States, the European Union and Britain say Russia now has the capability to act against Ukraine, and Johnson has warned of a catastrophe if Moscow were to do so.
"We urge Russia to step back and engage in dialogue to find a diplomatic resolution and avoid further bloodshed," Johnson said in remarks ahead of his arrival.
"It is the right of every Ukrainian to determine how they are governed. As a friend and a democratic partner, the UK will continue to uphold Ukraine's sovereignty in the face of those who seek to destroy it."
Johnson's visit reflects his desire to position Britain as a player in world affairs, even as he deals with a political scandal at home over gatherings at his offices and residence during COVID-19 lockdown that could force him from office.
Britain on Monday said any incursion would trigger unprecedented sanctions against Russian companies and people with close links to the Kremlin.
The government plans to target Russia's richest men include the ability to freeze their assets and to turn them back at the border.
Johnson will discuss with Zelenskiy what strategic support Britain can offer to Ukraine, according to a statement from his office.
Britain has supplied defensive weapons and training personnel to Ukraine, though ministers have said that the deployment of combat troops is unlikely.
British foreign minister Liz Truss was due to be on the trip but she tested positive for coronavirus late on Monday and is self-isolating at home.
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