Charles spoke as tensions between Europe and Russia appeared to intensity this week after President Vladimir Putin accused Kyiv's European allies of sabotaging US-led efforts to end the war.
The King's comments were made during a Windsor Castle state banquet for German President Frank-Walter Steinmeier - the first German head of state to make a formal state visit to Britain in 27 years.
Charles told guests seated in St George's Hall: "With regard to shared defence, our German-British Bridging Battalion at Minden - which I visited during our state visit two years ago - is literally building bridges, a unique partnership at the heart of NATO.
"And more broadly, of course, the United Kingdom and Germany together stand with Ukraine and bolster Europe against the threat of further Russian aggression."
Putin has rejected changes proposed by Ukraine and Europe to a draft peace plan as unacceptable, and he also said Moscow was "ready" for war with Europe.
Steinmeier and his wife Elke Budenbender were guests of honour at the dinner marking their three-day state visit while others present included German supermodel Claudia Schiffer and her filmmaker husband Sir Matthew Vaughn.
Charles told the banquet guests who included the Prince and Princess of Wales: "It is 36 years since the fall of the Berlin Wall - a momentous event in human history.
"The tumultuous period of political, social and technological change that happened after that has tested our values. Many have found that unsettling and even frightening. Fear can lead to anger and resentment.
"But the United Kingdom and Germany are united in a continued belief in democracy, freedom and the rule of law."
Steinmeier visited Prime Minister Keir Starmer on Wednesday and joined roundtable of UK and German business leaders.
"The leaders agreed on the importance of continuing to work together to deliver a just and lasting peace for Ukraine," Starmer's office said after the meeting.
The visit coincided with the announcement of a STG20 million ($A41 million) electric vehicle project led by Mercedes. It aims to boost the UK car industry by developing an ultra-compact electric drive system for high-performance EVs.
On Thursday, Steinmeier will address UK parliament and lay flowers on the tomb of the Queen Elizabeth II at St George's Chapel inside Windsor Castle.
with AP
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