The unnamed 75-year-old man died after being struck by terrorist Khalid Masood's car in Westminster on Wednesday.
A Met Police spokesperson said: "The man had been receiving medical treatment in hospital following the attack and life support was withdrawn this evening.
"Next of kin have been informed and are receiving support from specially trained family liaison officers."
He is the fourth victim to die in the atrocious attack, along with PC Keith Palmer, Aysha Frade and Kurt Cochran.
Kurt Cochran, from Utah, was was on a dream holiday around Europe with his wife Melissa who was seriously injured with a broken leg and rib in the outrage.
The American couple were on the final day of their trip, celebrating their silver wedding anniversary, when they were caught up in the carnage.
The pair were in the Capital visiting Mrs Cochran's parents who are serving as missionaries in the London Church of Jesus Christ of Latter-day Saints.
US president Donald Trump said Mr Cochran was a "great American" and said his "prayers and condolences are with his family and friends".
A GoFundMe page set up in aid of Mrs Cochran said that the couple were "self-employed, having built a recording studio business from the ground up over the last 10 years".
London terror attack victims PC Keith Palmer, Aysha Frade and Kurt Cochran
In a message posted on Facebook, his sister-in-law Shantell Payne said: “With a heavy heart I must pass the sad news of our beautiful brother, father, husband, son and friend Kurt Cochran.
“He could not overcome the injuries he received in the London terror attacks.
“This pain is so heart wrenching and raw it has rocked our family and all that knew him to its core.
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"My sister, Melissa, has a broken leg, a broken rib, and a cut on her head.
"While we are glad she survived, our hearts are broken and will never be the same after losing our dear uncle, brother-in-law, father. Kurt, you are a HERO, and we will never forget you."
London terror attack victims Thu, March 23, 2017
ISIS claims responsibility for London attack that killed at least three victims
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Ms Frade, a 43-year-old mother-of-two, was head of Spanish at DLD College, a private sixth form college based near the scene of the attack on Westminster Bridge.
It is thought she was on her way to collect her children aged eight and 11 from school when she was mowed down by the terrorist’s 4X4 Hyundai hire car.
According to Spanish media she was a British citizen and was a married to a Portuguese man called John Frade.
Her family hail from the town of Betanzos near the Galician city of La Coruna where her two sisters run a an English language school.
London terror attack victim Kurt Cochran was on a dream holiday from the US
Betanzos Mayor Ramon Garcia said the town was “terribly shocked” by Ms Frade’s murder.
He said: “This is a small town and we all know each other here.
“I would like to offer my condolences to Aysha’s families and the families of the other innocent victims who will obviously be suffering some very traumatic times and moments.”
A minute’s silence was observed outside the town hall at as a mark of respect to Ms Frade and the other victims of the Westminster attack.
Ms Frade's sisters Michelle and Silvia both live in the area and run an English language academy called Notting Hill in Betanzos.
They are travelling to Britain after informing pupils of the tragedy and telling them classes were being suspended until further notice.
London terror attack victim Aysha Frade
Tributes have been paid to PC Palmer who died after being attacked by the terrorist with a knife.
Scotland Yard's top anti-terror officer Mark Rowley said of his colleague: "He was someone who left for work today expecting to return home at the end of his shift – and he had every right to expect that would happen."
The Metropolitan Police Federation, which represents officers from the force, said: "PC Keith Palmer. Our brave Met Police colleague who made the ultimate sacrifice.”
London terror attack victim PC Keith Palmer
A chorus of thanks also came from politicians on all sides of the House.
Home Affairs Select Committee chairwoman Yvette Cooper, who was in the Palace of Westminster when the terrorist burst through the gates, said: "Thank you PC Keith Palmer – to your bravery many others owe their lives and safety."
Acting Deputy Commissioner Rowley said a total of 29 people had been treated in hospital with seven attack victims still in a critical condition.
The casualty list includes a number of foreign visitors who were sightseeing in the capital when the horror began to unfold.
Paramedics help a victim of the London terror attack
Three French teenagers in London on a school trip are among the injured and two of them are believed to be in a critical condition.
Five South Koreans were wounded in the London terror attack, according to Asian country's foreign ministry.
Three women and one man in their 50s and 60s suffered injuries including broken bones and another woman in her 60s suffered a head injury while falling and was taken to hospital.
A foreign ministry spokesman said four of the five were later discharged from hospital.
London Terror Attack: Latest Pictures Thu, March 23, 2017
4 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
A young Chinese woman suffered a broken shoulder and bruising when she was struck by the car and is receiving treatment for her injuries in hospital.
Chinese President Xi Jinping and Premier Li Keqiang have sent condolences to the United Kingdom over the attack.
Australian Foreign Minister Julie Bishop said an Australian woman was also hit by the speeding Hyundai on Westminster Bridge and is in hospital recovering from foot injuries.
The woman, who has not been named, is originally from Germany but is a permanent resident in South Australia.
The casualty list from the attack also includes a Pole, an Irish citizen, an Italian an American and two Greeks.
A woman receives treatment in the wake of the London terror attack
Italian ambassador to London Pasquale Terracciano said: "We tracked a Roman lady who was hit by the car driven by the terrorist.
“Scotland Yard had not reported it to us probably because she was taken to a hospital outside the area of the attack.
“She underwent surgery. She has a fractured ankle. The family has been informed, but for reasons of privacy her name was not released.
"Her partner was with her but managed to dodge the car."
London terror attack Travis Frain
Four British university students were also among the injured. They were in group of 13 students and a lecturer rom Edge Hill University in Ormskirk, Lancs, on a two-day trip to the Houses of Parliament.
Travis Frain, 19, Owen Lambert, 18, and two other students were taken to hospital as “walking wounded”.
Mr Frain is understood to have suffered a cracked rib and injuries to his hand and arm and will be undergoing surgery.
His nother Angela Frain, 45, said: "Travis said that all he could remember was walking along the bridge and the next thing he knew he was hit.
"He's a bit up and down and very shaken up by what's happened. He's due to go into surgery tonight and will be there for a few days.
"But I know he's extremely lucky and it could've been a lot worse.
"He's on a lot of drugs at the moment and so doesn't want to speak about what happened just now."