Trump joined other world leaders in northern France where thousands of well-wishers gathered to pay tribute to the ever-dwindling number of veterans of the famed landings which shaped the outcome of World War II.
"To all of our friends and partners, our cherished alliance was forged in the heat of battle, tested in the trials of war and proven in the blessings of peace. Our bond is unbreakable," Trump said in Colleville-sur-Mer.
The sober speech under bright blue skies also paid tribute to the men who braved Nazi bullets to take part in the biggest naval operation in history on June 6, 1944.
About 60 of them, many in wheelchairs and sporting baseball caps saying "World War II Veteran", sat in rows behind Trump and French President Emmanuel Macron, who at one point helped one of the seniors to his feet to receive applause.
"We know what we owe to you, veterans: our freedom," Macron told the crowd in English. "And on behalf of my nation I just want to say thank you".
In a pointed message targeting Trump's "America First" slogan, Macron added: "America is never as big as when it is fighting for the freedom of others."
D-Day is seen by many as a great symbol of transatlantic cooperation, with thousands of young American servicemen sacrificing their lives to end the Third Reich's grip on Europe.
By the end of what has become known as "the longest day", 156,000 Allied troops and 20,000 vehicles had landed in Nazi-occupied northern France on June 6 despite facing a hail of bullets, artillery and aircraft fire.
"The men behind me will tell you they are just the lucky ones," Trump said. "As one of them recently put it, all the heroes are buried here."
Thursday's commemorations follow a gathering in Portsmouth, southern England, on Wednesday attended by Trump, Macron, Queen Elizabeth II, German Chancellor Angela Merkel and Canadian premier Justin Trudeau.
Macron and British Prime Minister Theresa May laid the first stone for a new British memorial to fallen soldiers near the village of Ver-sur-Mer.
"Standing here as the waves wash quietly onto the shore below us, it's almost impossible to grasp the raw courage it must have taken that day to leap from landing craft and into the surf, despite the fury of battle," May said.
Many well-wishers have flocked to northern France to see the last living war veterans in the flesh.
"I attend the ceremonies every year, but the 75th anniversary is different, because we know that some of these people won't be around five years from now," Marcel Deschateaux told AFP in the town of Bayeux where Britain's Prince Charles attended a church ceremony.
Macron and Trump talk
"We must never stop working for the alliance of the free world," Macron told the ceremony.
Macron and Trump -- whose once warm relations have chilled due to mounting public disagreements on Iran, climate change and trade -- will next hold private talks, followed by a working lunch.
The last time Macron hosted Trump in France -- for the World War I centenary in November last year -- it turned into a diplomatic fiasco.
Days after the trip, Trump tweeted about Macron's "very low approval ratings" and reminded him how the French "were starting to learn German in Paris before the US came along" in World War II.
The two men appeared friendly Thursday, though the body language was cooler than during their first meetings, particularly when Trump was the guest of honour at celebrations of France's national day on July 14, 2017.
The US president planted a kiss on Macron's cheek as they greeted each other Thursday. Side-by-side the two men watched an airforce display and toured the Colleville military cemetery where 9,400 US servicemen are buried.
Macron will end the day with an homage at Colleville Montgomery for the Kieffer Commando, the only French soldiers to storm a Normandy beach on D-Day which opened a new front against the Nazis and led to the liberation of France and much of western Europe.
The 177 men, have long been little more than a footnote in France's official war history -- an oversight officials in Macron's office said he is eager to correct.
Trudeau will attend a ceremony at Juno Beach, where Canadian forces were in charge of the assault.
President Vladimir Putin, who was invited to the 60th anniversary of the invasion in 2004, did not receive an invitation this time, in a snub indicative of the West's strained relations with Russia.
Russia's foreign ministry said Wednesday the Allied invasion on D-Day did not determine the course of World War II, and its importance should not be exaggerated.
Spokeswoman Maria Zakharova told journalists it was the efforts of the Soviet Union, which entered the war in 1941, that secured victory.