Sir Keir’s spokesman said the party leader was feeling well and has not displayed any symptoms of coronavirus. He will be working from home.
It is the second time during the Covid-19 pandemic that Sir Keir has gone into self-isolation.
He put himself into quarantine in September after his daughter showed possible signs of coronavirus infection, but was able to resume normal activities after just two days when her test came back negative.
It is understood that the Labour leader was last in contact with the affected member of his private office staff on Tuesday. He will remain in self-isolation until 16 December, 14 days after his last contact.
His spokesman said: "This afternoon Keir Starmer was advised to self-isolate after a member of his private office staff tested positive for the coronavirus.
“Keir is well and not showing any symptoms. He will now be working from home.”
It is understood that other members of the Labour leader’s team are self-isolating.
This afternoon I was advised to self-isolate after someone who works in my office tested positive for coronavirus. I’m pleased to say my colleague is feeling ok.
I'm not showing any symptoms. But will now be working from home.
Keep safe and follow the public health advice.
— Keir Starmer (@Keir_Starmer) December 5, 2020
In September, Sir Keir missed a session of prime minister’s questions in the House of Commons, with deputy Angela Rayner stepping in to take his place.
However, following that occasion Commons speaker Sir Lindsay Hoyle agreed to allow Boris Johnson to take part in the weekly grilling by video link when the PM was forced to self-isolate.
Labour is yet to determine whether the same facility will be made available for Sir Keir.