A spokeswoman for the Prime Minister and his partner said both mother and baby are "doing very well" after the birth in a London hospital this morning (Wednesday, April 29).
The spokeswoman said: "The Prime Minister and Ms Symonds are thrilled to announce the birth of a healthy baby boy at a London hospital earlier this morning. Both mother and baby are doing very well.
"The PM and Ms Symonds would like to thank the fantastic NHS maternity team."
Ms Symonds revealed her engagement to the Prime Minister and that she had been expecting a child with him on Instagram on February 29.
The Prime Minister is famously guarded about his children and has previously declined to say how many he has.
The 55-year-old has four children with his wife Marina Wheeler, who he has since separated from, but he is believed to have fathered a number of children outside his marriage.
During an election campaign appearance on LBC Radio in November last year, Mr Johnson was asked about his children by host Nick Ferrari.
The PM Said: "I love my children very much, but they are not standing at this election, and I am not therefore going to comment."
The birth comes just days after his return to Downing Street after spending the past two weeks at the Chequers estate recovering from coronavirus.
The Prime Minister resumed work on Monday (27 April), chairing the government’s morning meeting of the coronavirus “war cabinet”.