Johnson held talks with his Polish counterpart Mateusz Morawiecki and also met with President Andrzej Duda to discuss the importance of a united front in the face of Russian aggression, according to officials.
"The reason I'm here is because Poland is absolutely critical to our collective European security and the lesson of the last 100 years is that when Poland is threatened with instability, or aggression on the borders of Poland, then we are all threatened and we're all affected," Johnson said.
"And I just want to be absolutely clear, when Poland is threatened then the UK stands ready to help, as indeed Poland has always stood ready to help the United Kingdom," he added.
Poland's Morawiecki said that Johnson's visit was a sign of allied solidarity in the face of threats to European security.
Morawiecki thanked the UK for showing "great solidarity with Ukraine, Poland and the whole eastern part of NATO," Polish state news agency PAP reported.
He also said that "Poland and the UK are unequivocally on the side of peace and security."
'Putin's political goal is to break up NATO': Polish PM
Speaking at a joint news conference with Johnson, Morawiecki said that Russian President Vladimir "Putin's political goal is to break up NATO."
"That's why we need to stay close together," he added.
He told reporters that "there is a great understanding" between Poland and Britain "when it comes to the tensions on NATO's eastern flank."
Morawiecki also said that “on one side of the scales, there is freedom and European security; on the other side, there is destabilization and the things that the Kremlin is cooking up."
He added: “We want to build bridges and connections with Russia, but we want to build them on the foundation of peace."
While in Poland on Thursday, Johnson also visited British soldiers stationed in the country, with Polish leaders thanking him for the UK’s support to Poland’s security.
Earlier in the day, a contingent of British Royal Marines arrived in Poland under an agreement between the two governments, officials announced.
Polish Defence Minister Mariusz Błaszczak said the arrival of the soldiers amid a Russian military buildup near Ukraine was "an expression of solidarity" in the face of "the Kremlin's aggressive policies."
He added that British troops stationed in Poland "significantly support the security of the eastern flank of NATO."
The Royal Marines arrived after UK Defence Secretary Ben Wallace said earlier this week that Britain would send an additional 350 troops to Poland.
The UK defence ministry's press office said in a statement at the time that 350 Royal Marines would deploy to Poland to support the country "with joint exercises, contingency planning and capacity building as the build-up of Russian forces on Ukraine’s border continues."
Britain last year sent 100 troops to help Poland deal with a migrant crisis at its border with Belarus.
'Combination of sanctions and military resolve, plus diplomacy'
Before his trip to Warsaw, Johnson earlier on Thursday visited Brussels to meet with NATO Secretary-General Jens Stoltenberg.
Speaking at a joint news conference with Stoltenberg, Johnson said that the West's standoff with Russia over Ukraine "is the biggest security crisis that Europe has faced for decades."
He added: "And I think that the combination of sanctions and military resolve, plus diplomacy are what is in order."
Boris Johnson and Jens Stoltenberg meet in Brussels on Thursday. Photo: EPA/OLIVIER HOSLET
Johnson last week expressed deep concern about Russia's hostile activity on the Ukrainian border in a call with Russian President Vladimir Putin, the BBC reported.
Johnson warned Putin that any further incursion by troops on the border would be "a tragic miscalculation" by Russia, according to the British public broadcaster.
Source: PAP, Reuters
Click on the audio player above for a report by Radio Poland's Michał Owczarek.