The annual tradition marks his official birthday and includes a military parade, music, a gun salute and Royal Airforce (RAF) flyover.
Crowds began to gather to watch the parade in central London as it makes its way along the Mall, near Buckingham Palace, with thousands expected to attend.
This year the King has asked the Royal Family taking part in the parade to wear black armbands, as a mark of respect to the victims of the Air India plane crash, Buckingham Palace said.
The King requested a minute's silence, which will come after his inspection of the parade, in recognition of "the lives lost, the families in mourning and all the communities affected by this awful tragedy", a Palace spokesman said.
The King had sent a message of support soon after the news broke of the air crash, which claimed the lives of all but one of the 242 passengers and crew.
Flags have been at half-mast at royal residences and the black armbands will add another sign of respect, with the King having said he was "desperately shocked by the terrible events in Ahmedabad".
Coachmen and women on carriages in the parade are also likely to wear black armbands, along with those senior royals in military uniform in the parade, including the King and the Prince of Wales.
Those royals at the ceremony but not in uniform will probably not wear armbands.
Today’s pageantry kicks off at 10:45 BST at Horse Guards Parade, with the event’s route going along the Mall near Buckingham Palace before turning back.
Those heading down to central London, will get an opportunity to view the military spectacle by standing on the Mall or on the edge of St James’s Park overlooking Horse Guards Parade.
The event is due to finish at around 12:25 BST but there’s more to come afterwards - the Royal Airforce Flypast over central London will take place at 13:00 BST.
To spot the royals, the flypast will be one of the best moments as they traditionally stand on the balcony of Buckingham Palace to see it.
This will be a Trooping the Colour with a very different feel to last year, when there was feverish interest in the return to public life for Catherine, the Princess of Wales.
Trooping the Colour in 2024 had been her first public appearance of the year after her cancer diagnosis, and it was a moment that seemed full of uncertainty.
Catherine is now in remission and when she appears today the attention on her health will be much less intense.
King Charles is still receiving cancer treatment and will travel in a carriage, as he did last year, rather than riding on horseback.
But he seems to be well enough for a full diary of engagements and is recently back from what seemed to have been a very successful trip to Canada, where he showed support for a country feeling under pressure from US President Donald Trump.
King Charles’ actual birthday is 14 November, but Trooping the Colour has marked the monarch’s birthday in June for more than 260 years.
This year it’s the Number 7 Company Coldstream Guards trooping their flag - also known as their “colour” - in the presence of the King, with more than 1,350 soldiers of the Household Division of the British Army expected to take part in today’s parade.
There’ll be music to march along to, royals to spot, and an RAF flypast zooming across the skies over central London near Buckingham Palace.
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