The 33-year-old actor, who grew up in the suburbs of Belfast, announced the news in an exclusive interview with People magazine this week.
The pair, who have been married since April 2013, will be adding to their family as they already have a two-year-old daughter, Dulcie.
Jamie rose to fame in BBC drama The Fall alongside Gillian Anderson, set in his native Northern Ireland, and has since become a Hollywood heartthrob in the film Fifty Shades of Grey.
Daughter Dulcie was born just three days before the actor began shooting his biggest role to date as EL James' character Christian Grey.
Reflecting on the busy time, Jamie said; 'It was an intense shoot so you can only imagine how mad that time was... changing nappies, having a little baby vomit on my shoulders and waking up four times a night.'
The former Abercrombie & Fitch model proposed to wife Amelia in November 2012, just five months before their big day at the romantic Orchardleigh House in Somerset.
Jamie said; 'I was smitten from the start... I'm more in love with her today than I was at the start.'
Speaking about becoming a family man, Jamie explained how his career doesn't affect his home life; 'I mean, we live a pretty quiet life. My wife and I and our baby, we're not massively aware of it, we just go about our day.'
'We're not chased down the street or anything!' Jamie had a series of high-profile relationships before he settled down with Amelia, dating English actress Kiera Knightly between 2003 and 2005.
He has also been linked to English beauty Sienna Miller and American actress Mischa Barton, who starred in cult teen-program The OC.