A spokesperson for the couple said: "The Prime Minister and Miss Symonds are very pleased to announce their engagement and that they are expecting a baby in the early summer."
Mr Johnson, 55, and Miss Symonds, 31, made history as the first unmarried couple to officially live together in Downing Street when they moved in last year.
Ms Symonds, a conservationist and former Conservative Party communications chief, first found herself making headlines when she was romantically linked to Mr Johnson earlier in 2019.
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But her association with Mr Johnson dates back to when she worked on his successful re-election bid at City Hall in 2012.
An early summer birth would suggest the new arrival was conceived during the Autumn, around the time that the October 31 Brexit deadline was extended.
Mr Johnson's union with Miss Symonds will be his third marriage.
Earlier this month, a court heard that the Prime Minister and his estranged second wife Marina Wheeler were preparing to end their marriage after reaching an agreement over money.
A family court judge approved a financial settlement and gave Ms Wheeler permission to apply for a divorce decree.
Mr Johnson is believed to be the first prime minister to be divorced in office in modern times.
Ms Wheeler and Mr Johnson, who have four children, separated in 2018 after marrying in 1993.
The couple announced that they had separated in September 2018.
He was a childhood friend of Ms Wheeler - the daughter of BBC journalist Charles Wheeler - when both were pupils at the European School in Brussels.
Mr Johnson met his first wife, Allegra Mostyn-Owen, while they were students at Oxford.
They married in 1987, but the marriage was annulled in 1993.
In 2013 it emerged during another court hearing that Mr Johnson had fathered a daughter during an adulterous liaison while Mayor of London in 2009.
In 2004, he was sacked from the Tory frontbench over a reported affair with journalist Petronella Wyatt.