Health officials said the child went to a school in Ealing.
Dr Yimmy Chow, health protection consultant at the UK Health Security Agency, said: "We are extremely saddened to hear about the death of a child at St John's Primary School, and our thoughts are with their family, friends and the school community.
“Working with Ealing Council public health team, we have provided precautionary advice to the school community to help prevent further cases and we continue to monitor the situation closely."
"In rare incidences, it can be a severe illness and anyone with high fever, severe muscle aches, pain in one area of the body and unexplained vomiting or diarrhoea should call NHS 111 and seek medical help immediately," Mr Chow added.
Two other schoolchildren elsewhere in the country have died from the infection in the past seven days.
One is a pupil from Victoria primary school in Penarth, four miles south of Cardiff.
Last week, a six-year-old died after an outbreak of the same infection at a school in Surrey.
The parents of a four-year-old boy from Buckinghamshire have paid tribute after he died on November 14 after contracting a Strep A infection.
Shabana Kousar, the mother of Muhammad Ibrahim Ali, who attended the Oakridge school and nursery in High Wycombe, told the Bucks Free Press: “The loss is great and nothing will replace that. He was very helpful around the house and quite adventurous, he loved exploring and enjoyed the forest school, his best day was a Monday and [he] said how Monday was the best day of the week.
“He also had a very close bond with his dad. He was his best friend and went everywhere with him. He just wanted to be with him.”
He was described as a “wonderful, kind, smiley and energetic boy” on a JustGiving page set up in his memory.
Group A strep bacteria can cause many different infections, ranging from minor illnesses to deadly diseases.
Scarlet fever is caused by Strep A and mostly affects young children but is easily treated with antibiotics.
The first signs of scarlet fever can be flu-like symptoms, including a high temperature, a sore throat and swollen neck glands
A rash appears 12 to 48 hours later which starts on the chest and stomach, then spreads.
A white coating also appears on the tongue which peels, leaving the tongue red, swollen and covered in little bumps (often called “strawberry tongue”).
Dr Simon Clarke, microbiologist at the University of Reading, said: “It strikes me that as we are seeing with flu at the moment, lack of mixing may have caused a drop in population wide immunity that could increase transmission, particularly in school age children.”