Trump and his wife Melania arrived at Windsor Castle, the oldest and largest inhabited castle in the world and family home to British monarchs for almost 1000 years, where the royal red carpet treatment includes a carriage procession, gun salutes, a military flypast and lavish banquet.
Britain says it will be the largest military ceremonial welcome for a state visit in living memory.
Trump, an overt royal fan, has made little secret of his delight at being not just the first US leader, but the first elected politician to be invited by a British monarch for two visits.
On his arrival, the president told reporters he loved Britain.
"It's a very special place," he said.
Prime Minister Keir Starmer is hoping to use that sentiment to Britain's advantage as his government seeks to cement the two nations' "special relationship", deepen economic ties, secure billions of dollars of investment, discuss tariffs and press the US president on Ukraine and Israel.
The visit has already delivered a new technology pact between the two countries with companies from Microsoft to Nvidia, Google and OpenAI pledging STG31 billion ($A63 billion) in British investments over the next few years, in AI, quantum computing and civil nuclear energy.
But while Starmer is banking on the unrivalled royal allure helping cajole the anglophile president, many pitfalls for British prime minister remain.
Polls show Trump is widely unpopular in Britain and Starmer, faced with plummeting poll ratings of his own and economic woes, will need to show that playing his royal trump card will reap benefits.
Awkward questions about late sex offender Jeffrey Epstein could also come to the fore.
Starmer, last week, sacked Peter Mandelson as Britain's ambassador in Washington over his ties to Epstein, which could lead to questions for both Starmer and Trump, whose own relationship with the financier has also come under scrutiny.
Large protests are also expected in London on Wednesday with 1600 officers deployed to deal with a demonstration by the "Stop the Trump Coalition".
Wednesday will be dominated by ceremony. Charles and his wife Queen Camilla joined the Trumps on a carriage procession through the grounds of the castle, with the route lined by 1300 British service personnel.
The royals will later show the president and first lady historical items from the Royal Collection relating to the US, before the Trumps visit St George's Chapel, the final resting place of Queen Elizabeth, who hosted Trump for his first state visit in 2019, where he will lay a wreath on her tomb.
Later there will be a flypast by military aircraft before the state banquet.
The trip will continue on Thursday at  Starmer's Chequers country residence where the pair are expected to give a joint news conference.
The president returns to Washington on Thursday night.
