First Minister Arlene Foster has congratulated Prime Minister Boris Johnson and Carrie Symonds on the birth of their baby son.
She said there would be more sleepless nights ahead for the couple.
"Wonderful news of the birth of a baby boy! Many congratulations to the PM and Carrie Symonds," she tweeted.
UUP leader Steve Aiken also tweeted his congratulations
"We wish them, and their son, all the best (& to pass on our wishes to all other newborns & parents as well)," he added in a post on Twitter.
The baby was born in a London hospital early on Wednesday.
Both mother and baby were doing "very well," a spokesman said.
The birth comes just weeks after the PM was discharged from intensive care where he had been battling coronavirus. He returned to work on Monday and spoke of how the virus felt like an "invisible mugger".
Ms Symonds also suffered symptoms of the disease.
Mr Johnson has previously suggested he intends to take paternity leave, although it is not known if this remains the case given the coronavirus crisis.
The Prime Minister's spokeswoman said the couple were "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."
Mr Johnson, 55, and Ms 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 Prime Minister will not appear at Prime Minister's Questions in the Commons on Wednesday afternoon.