British Prime Minister was contacted by NHS Test and Trace on Sunday but is not showing symptoms, Downing Street said.
Johnson spent about 35 minutes with Ashfield MP Lee Anderson who has since tested positive, BBC political correspondent Nick Eardley said.
In April, Johnson spent three nights in intensive care after falling ill with the virus.
He later said it “could have gone either way” and thanked healthcare workers for saving his life.
In a statement, a No 10 spokesman said: “The prime minister has today been notified by NHS Test and Trace that he is required to self-isolate as a contact of someone who has tested positive for Covid-19.
“The prime minister will follow the rules and is self-isolating.
“He will carry on working from Downing Street, including on leading the government’s response to the coronavirus pandemic.
“The PM is well and does not have any symptoms of Covid-19.”