Rau held talks with his Ukrainian counterpart Dmytro Kuleba during a visit that officials in Warsaw described as "extraordinary and urgent" and "associated with a threat to peace at Ukraine's borders."
"Ukraine has the right to defend itself," Rau said, as cited by the Polish foreign ministry.
He added: "Ukraine is not isolated in the defence of its sovereignty, territorial integrity and the inviolability of its borders."
'Threats to peace in Europe'
The foreign ministry in Warsaw said that Rau and Kuleba discussed the "threats to peace in Europe resulting from the escalation of tensions along the northern and eastern border of Ukraine and in occupied Crimea, and from the construction of Nord Stream 2," the contested gas pipeline being built from Russia to Germany.
Meanwhile, Kuleba wrote on Twitter that it was "important to enhance the Ukraine-Poland strategic partnership" as Russia moved to "elevate security threats for Ukraine, Europe & the Transatlantic community."
He added: "Poland is our long-standing ally & trusted friend."
The Polish foreign minister's trip to Kiev came after what public broadcaster Polish Radio described as destabilizing moves by Russia, the escalation of tensions in eastern Ukraine, the concentration of Russian troops near the border with Ukraine, and an intensification of Russian propaganda.
Earlier this week, Ukrainian President Volodymyr Zelenskiy called on NATO to lay out a path for his country to join the Western military alliance, after days in which Russia has massed troops near the conflict-hit Donbass region, the Reuters news agency reported.
The Donbass conflict erupted after Russian forces seized Ukraine's Crimean peninsula in 2014. Ukraine and Western countries say Donbass separatists have been armed, led, funded and aided by Russians, including active Russian troops, Reuters noted.
Kuleba said his meeting with Rau had originally been scheduled for a later date. "But due to the situation, Zbigniew Rau amended his plans and came to Kiev sooner, at my invitation."