"The Prime Minister and Ms Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning,"a spokeswoman said."Both mother and baby are doing very well.
"The PM and Ms Symonds would like to thank the fantastic NHS maternity team."
Johnsonreturned to work on Monday,a month after testing positive for COVID-19 which he said had threatened his life. Symonds also had symptoms of the virus but recovered swiftly.
"Great to hear Downing Street is getting a new resident,"said Johnson's Finance Minister,Rishi Sunak.
Opposition Labour Party leader Keir Starmer said:"Wonderful news. Many congratulations to Boris Johnson and Carrie Symonds."
Australian Prime Minister Scott Morrison sent his congratulations to the couple on Twitter,saying there's"nothing better than being a dad".