Carrie Symonds, 31, announced on her private Instagram account that she got engaged to Prime Minister Johnson, 55, at the end of last year.
Symonds wrote, "I wouldn't normally post this kind of thing on here, but I wanted my friends to find out from me... Many of you already know but for my friends that still don't, we got engaged at the end of last year... and we've got a baby hatching early summer. Feel incredibly blessed." They are yet to announce the wedding date.
This makes Johnson the first prime minister to tie the knot in office since the Earl of Grafton in 1769.
The prime minister this month concluded his divorce from ex-wife Marina Wheeler with whom he has four children.
Carrie Symonds currently a senior adviser for US environmental campaign group Oceana, after quitting her role with the Tories in 2018. She moved into No. 10 Downing Street with prime minister Johnson last in July.