He wrote on Twitter: “This morning I was notified that I must self isolate after coming into contact with someone who has tested positive for Covid-19.
“I have no symptoms and will be working from home until next Monday.”
It is not yet confirmed whether the latest stay-at-home notice means the Opposition leader will miss his regular slot at Prime Minister’s Questions with Boris Johnson on Wednesday.
It is the third time the Labour leader has had to go into isolation.
In December Sir Keir had to self isolate for 14 days after a member of his staff tested positive for coronavirus. In September, a member of his family showed symptoms of the virus.
But they later tested negative, allowing Sir Keir to get back to Westminster.
Anyone contacted by track and trace is required to isolate until 10 days after they came into contact with the positive Covid case.
With rules requiring him to quarantine until next Monday, his close contact is likely to have taken place on Friday January 22.
It was not revealed how Sir Keir came into that close contact.
Last week Health Secretary Matt Hancock had to go into self-isolation after getting an NHS alert.