Rubio was addressing the annual Munich Security Conference on Saturday, where Europe's leading powers have tried to project their own independence and strength while straining to keep a long-time alliance with the US under President Donald Trump alive.
The speech delivered a degree of respite to European countries who fear being left in the lurch on anything from the war in Ukraine to international trade ructions in a rapidly shifting global order.
But it was short on concrete commitments and made no mention of Russia, raising questions about whether Rubio's tone, more emollient than that of Vice President JD Vance at the same event a year ago, would change the underlying dynamics.
"In a time of headlines heralding the end of the transatlantic era, let it be known and clear to all that this is neither our goal nor our wish, because for us Americans, our home may be in the Western Hemisphere, but we will always be a child of Europe," Rubio said.
"For the United States and Europe, we belong together," he said in the speech, which drew a standing ovation at the end.
While European Commission President Ursula von der Leyen said she was "very much reassured" by the speech and German Foreign Minister Johann Wadephul called Rubio a "true partner", others struck a more cautious tone.
"For most Europeans, the common interest is security," said Gabrielius Landsbergis, former foreign minister of NATO member Lithuania.
"This was not a departure from the general position of the (Trump) administration. It was simply delivered in more polite terms," he said on X.
One particular area of anxiety is Ukraine, where allies have long worried about Trump and Russia's Vladimir Putin trying to ram through a deal on Moscow's terms and force Kyiv to cede land to end Europe's deadliest conflict since World War Two.
On Saturday, five European allies accused Russia of killing Kremlin critic Alexei Navalny using venom from poison dart frogs while he was held in an Arctic penal colony two years ago. Russia has repeatedly denied responsibility for his death.
US-brokered peace talks resume next week in Geneva after a sustained bombardment of Ukrainian cities during one of the coldest winters in years killed civilians and left hundreds of thousands of people without power and water.
President Volodymyr Zelenskiy expressed hope at the conference on Saturday but said he was feeling "a little bit" of pressure from Trump, who had said Zelenskiy should not miss the "opportunity" to make peace soon.
"The Americans often return to the topic of concessions and too often those concessions are discussed only in the context of Ukraine, not Russia," Zelenskiy said.
The Munich conference of top security leaders has been dominated this year by how countries are scrambling to adjust to a year of confrontations with Trump on issues from tariffs to his threat to wrest Greenland from fellow NATO member Denmark.
Last year's address by Vance, a potential rival to Rubio in the 2028 US presidential race, dressed down European allies, arguing the greatest danger to Europe came from censorship and democratic backsliding rather than external threats like Russia.
While praising Europe's cultural achievements from the artist Michelangelo to the poet William Shakespeare, Rubio also touched on themes that have raised hackles, including criticism of mass migration and zealous action on climate change.
"We do not want our allies to be weak, because that makes us weaker," he said.
