The spokeswoman added, "The PM and Symonds would like to thank the fantastic NHS maternity team.". This news came as a surprise because Symonds was not thought to be due for several weeks, but she was said to be doing "very well".
Johnson, 55, and Symonds, 32, announced in March that they were expecting a baby in "early summer", and that they had become engaged at the end of last year.
The birth of the baby boy comes just days after Johnson, 55, returned to work after being admitted to hospital with coronavirus, including three nights in intensive care.
On Monday Boris Johnson returned to work, after a struggle with coronavirus. His partner Symonds also suffered symptoms of the disease. The Conservative colleagues of Johnson have been congratulating the couple on social media.
The Health Secretary Matt Hancock stated, "So thrilled for Boris and Carrie. Wonderful to have a moment of unalloyed joy!"
Sir Keir Starmer, the Labour leader also extended his congratulations on the "wonderful news".
Nicola Sturgeon the Scottish First Minister tweeted, "Some good news - sending congratulations to Carrie and the PM. And wishing health and happiness to the wee one".
The First Minister of Northern Ireland Arlene Foster also congratulated the couple, adding: "More sleepless nights ahead!"
There was speculation that Johnson would take part in his first Prime Minister's Questions later on Wednesday. But his place will be taken by Foreign Secretary Dominic Raab, who has been deputizing for him.
The British Prime Minister is believed to have at least five children, including four with his second wife, Marina Wheeler with whom he split in the year 2018.