The Irish mixed martial arts fighter confirmed the news on Instagram this afternoon, posting a smiling photo of the couple featuring an engagement ring and a teary-eyed Ms Devlin.
"What a birthday, my future wife!" the 32-year-old former Ultimate Fighting Championship featherweight and lightweight champion wrote.
The couple have been together for more than a decade since 2008 after first meeting at a nightclub, and share two children.
Ms Devlin is a native of Walkinstown, Dublin and featured as a major part of Mr McGregor's film Notorious, which followed his rise to UFC success.
Ms Devlin is hired to Mr McGregor's business and travels the world alongside the fighter.
The couple welcomed their first child in 2017, naming him Conor Jr.
Ms Devlin later announced her second pregnancy on Mr McGregor's 30th birthday in July of 2018.