The Labour leader was told he must enter quarantine just minutes after sparring with Boris Johnson at PMQs.
The prime minister will have avoided any contact with the virus during PMQs as he was participating remotely from Chequers after being told himself to isolate last week.
Sir Keir Starmer said: "This lunchtime, one of my children tested positive for Covid.
"I know it's my responsibility to follow the rules - my family and I are now self-isolating."
A spokesman said the Labour leader "was already doing daily tests and tested negative this morning. He will continue to take daily tests."
Prime Minister Johnson was told to self-isolate after being in contact with Health Secretary Sajid Javid who revealed on Saturday that he too had tested positive.
It means three of the UK' most senior politicians are in self-isolation, joining hundreds of thousands around the country who have been told to quarantine.
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This is the fourth time Sir Keir has had to enter quarantine since the pandemic began and will force him to alter plans to launch his "safer communities" campaign on Thursday.
Ironically, just minutes before being told to self-isolate, Sir Keir accused the PM of causing a "summer of chaos", criticising the confusion caused by his policy for self-isolation.
A surge in people being told to self-isolate is being driven by the UK's soaring coronavirus cases, with more than 328,000 new infections being recorded in the past week.
Earlier this week the government caused confusion when a minister said being pinged by the NHS app did not mean someone was not legally required to self-isolate.
But Number 10 released a statement afterwards saying it was "crucial" for people to self-isolate and they should continue to do so.
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