The Labour leader was said to have tested negative on Wednesday morning ahead of his appearance in Parliament where he grilled Boris Johnson over his isolation policy.
It will be 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.
A spokesman said: "One of Keir’s children tested positive for Covid this lunchtime. In line with the rules, Keir and his family will now be self-isolating.
"Keir was already doing daily tests and tested negative this morning. He will continue to take daily tests.”
Just after midday, Sir Keir accused the Prime Minister of ushering in a "summer of chaos", criticising the confusion caused by his policy for self-isolation as Covid-19 cases soar.
Mr Johnson responded virtually via a screen in the Commons while in isolation at his country retreat, Chequers, following contact with Covid-positive Health Secretary Sajid Javid.
Sir Keir’s statement announcing that he will swiftly enter quarantine with his family will doubtless be contrasted with the Prime Minister’s own approach over the weekend.
Mr Johnson and Chancellor Rishi Sunak initially tried to get around quarantine after being contacted by NHS Test and Trace by saying they would join a daily workplace testing programme.
But they were forced to back down on the plans and enter isolation amid widespread public anger that they would be free while tens of thousands of people are forced to miss work or school over coronavirus contacts.
Addressing the television screen in the Commons chamber on Wednesday, Sir Keir accused Mr Johnson of having tried to “dodge” quarantine.
The Labour leader kicked off a summer out on the road with calls for skilled jobs to revitalise Blackpool during a walkabout in the resort on July 15.
He held a series of meetings and events across Blackpool during his two visit to launch Labour proposals, including a £15bn children’s recovery plan.
His proposals included breakfast clubs and small group tutoring to help children catch up, mental health support in schools and an extension of free school meals.
Speaking ahead of his visit, Mr Starmer said: "I've got plans to create opportunities for everyone, and I’m looking forward to debating them with local people in Blackpool.
"I’ll be having robust conversations with people, particularly those who didn’t vote Labour at the last election."
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