The King and Queen Camilla were cheered and applauded by crowds after arriving in the town in south-west France on Friday.
Queen Camilla, who wore a turquoise Anna Valentine coat dress, earrings and the late Queen Elizabeth II's diamond brooch, was handed a bouquet as the royal couple greeted hundreds of well-wishers in the town which is twinned with Bristol.
Locals waved French and Union flags, and shouted “God save the king”.
Butcher Georges Britouille, 57, said: “This, for us, is historic, to have the king visit where we live is something to be remembered.”
King Charles and Queen Camilla met the Mayor of Bristol Marvin Rees and his Bordeaux counterpart Pierre Hurmic before being shown an agreement between the two cities.
They signed the town hall's guest book before going out to the garden to plant a loquat leaf oak tree.
King Charles giggled as Queen Camilla poured water on the plant, saying “very good”.
They later joined a reception on a royal navy frigate to celebrate military ties between the countries.
The town – an English possession in the Middle Ages that English and French royalty fought over for centuries – is home to around about 39,000 Britons today, and the king and queen met with Britons running businesses in the region.
After flying in to Bordeaux from Paris, the king and queen will briefly switch to more environmentally friendly public transport, riding a pioneering electric tram to the main city square.
The monarch met emergency workers and communities affected by wildfires in the region last year, as well as visiting an experimental forest designed to monitor the effects of climate change on urban woodlands while in Bordeaux.
The couple also travelled to a vineyard known for its sustainable approach to wine making, in a region where wine exports are a pillar of the economy.
Severe drought last year forced Bordeaux's earliest harvest yet, and it has long been working to adapt to climate change.
Bordeaux's Green Mayor Pierre Hurmic said he had no problem finding a common language with King Charles.
“We spoke very simply, naturally. In both French and in English. But our common language is ecology,” he said.
The visit comes months after vast swathes of Europe, including France, where thousands of people were evacuated, suffered devastating wildfires fuelled by climate change.
Friday’s events mark the third and final day of a state visit aimed at shoring up the alliance between Britain and France. His warm words towards France were met with a standing ovation in the Senate and even cheers of “Long Live the King!”
During the speech, when he became the first British monarch to address the chamber, he urged France to unite with Britain in tackling climate change.
“Just as we stand together against military aggression, so must we strive together to protect the world from our most existential challenge of all – that of global warming, climate change and the catastrophic destruction of nature,” he said.
“Jacques-Yves Cousteau said so wisely: ‘For most of history, man has had to fight nature to survive; in this century he is beginning to realise that, in order to survive, he must protect it’.”
King Charles III visits France – in pictures
In an address to the French Senate on Thursday, King Charles praised France and the United Kingdom’s “indispensable relationship” and its capacity to meet the world’s challenges, including Russia’s war in Ukraine and climate change. He called for a new “entente for sustainability”.
He also spoke about his concern for the climate in his toast at a state dinner in the Palace of Versailles on Wednesday evening.
The king’s comments came after Prime Minister Rishi Sunak announced he was watering down some of Britain’s climate commitments, including delaying the introduction of a ban on new gas and diesel cars from 2030 to 2035.
On Thursday, the second day of his state visit to France, the king met young athletes in a working-class, multicultural suburb of Paris and made an emotional stop in front of the fire-damaged Notre Dame Cathedral.