A man who was taking the photo then comforted her and the pair walked away while crowds of other visitors watched on.
The Household Cavalry soldier stayed motionless and expressionless during the incident, in line with protocol.
The woman, who has not been named, touched the horse despite a sign nearby warning visitors not to do so, adding: “Beware! Horses may kick or bite.”
It comes after armed police officers, who are typically stationed near the horses, arrested a TikTok prankster who was poking a microphone beneath the nose of a Household Cavalry horse.
The man was escorted away in handcuffs after he began arguing with tourists who confronted him for getting in the way of their picture.
Another female tourist was bitten by a horse in the King’s Guard earlier this year while posing for a photo, with the animal clenching her puffer jacket with its jaw after she attempted to stroke it.
In this case, the horse did not let go and shook its head violently while gripping on to her jacket as she tried to step away. She was not hurt but was left with a torn jacket.
And in September last year, a Household Cavalry soldier broke his customary composure to shout, “Do not touch the reins,” when a male tourist latched on to them before walking away in embarrassment.
Another royal mount was embroiled in another incident in April when four horses rampaged through central London after being spooked by noisy construction work during a morning ride.
The animals threw their riders, Life Guards and Blues and Royals servicemen, while they were leaving Hyde Park Barracks during preparations for rehearsals for the King’s birthday parade in June.
The Household Cavalry is formed of the most senior regiments in the British Army, with the Mounted Regiment acting as a ceremonial face representing the monarch and nation in state and public duties, alongside a separate operational armoured reconnaissance regiment.
The mounted regiment is tasked with protecting the King and while popular for tourist photo seekers, they are generally not allowed to interact with the public and must remain focused at all times.
Horse Guards Parade is named after the troops who have mounted The King’s Life Guard there since the Restoration of King Charles II in 1660.