He captioned the pic, "Right then. My Mum bought me this. Time to get ready! #trewlit #bookstagram #parenting."
Brand and his leading lady have dated off and on since 2007. After Brand divorced Katy Perry in July 2012, he and Gallacher, 27, a lifestyle blogger, got back together in 2015.
Congrats to the soon-to-be parents!
Russell Brand through the years
Russell Brand through the years
British comedian Russell Brand performs at his Messiah Complex show at Brixton Academy in London March 9, 2014. REUTERS/Olivia Harris (BRITAIN - Tags: ENTERTAINMENT)
SYDNEY, AUSTRALIA - OCTOBER 24: Russell Brand performs during his Trew World Order tour at Qantas Credit Union Arena on October 24, 2015 in Sydney, Australia. (Photo by Cassandra Hannagan/WireImage)
LONDON, ENGLAND - JUNE 20: Comedian Russell Brand speaks to thousands of demonstrators gathered in Parliament Square to protest against austerity and spending cuts on June 20, 2015 in London, England. Thousands of people gathered to march from the City of London to Westminster, where they listened to addresses from singer Charlotte Church and comedian Russell Brand as well as Len McCluskey, general secretary of Unite and Sinn Fein's Martin McGuinness. (Photo by Mary Turner/Getty Images)
British comedian Russell Brand speaks to protesters following a march against the British government's spending cuts and austerity measures in London on June 20, 2015. Thousands of demonstrators staged an anti-austerity march in London today, in the first major public protest since Conservative Prime Minister David Cameron won a general election. AFP PHOTO / JUSTIN TALLIS (Photo credit should read JUSTIN TALLIS/AFP/Getty Images)
LONDON, ENGLAND - APRIL 21: Russell Brand attends 'The Emperor's New Clothes: An Evening With Russell Brand' at Hackney Picturehouse on April 21, 2015 in London, England. (Photo by David M. Benett/Getty Images)
LONDON, ENGLAND - MARCH 26: Russell Brand opens the Trew Era Cafe on March 26, 2015 in London, England. The Trew Era Cafe is a social enterprise community project on the New Era estate. Russell Brand will be donating 100% of his money for his book 'Revolution' to The Trew Era Cafe. (Photo by Mike Marsland/WireImage)
LONDON, ENGLAND - MARCH 09: Russell Brand (L) and Boris Becker attend a party hosted by Instagram's Kevin Systrom and Jamie Oliver. This is their second annual private party, taking place at Barbecoa on March 9, 2015 in London, England. (Photo by David M. Benett/Getty Images for Instagram)
LONDON, UNITED KINGDOM - DECEMBER 05: Russell Brand meets fans and signs copies of his book 'Revolution' at Waterstone's, Piccadilly on December 5, 2014 in London, England. (Photo by Joseph Okpako/WireImage)
LONDON, ENGLAND - DECEMBER 01: Comedian Russell Brand joins residents and supporters from the New Era housing estate in East London as they deliver a petition to 10 Downing Street during a demonstration against US investment company Westbrook's plans to evict 93 families on December 1, 2014 in London, England. The protestors have taken their demonstration to Westbrooks Headquarters in Mayfair and plan to present a petition to the Prime Minister. Westbrook's are said to be planning to evict the tenants, refurbish the estate and re-let the flats at full market value (Photo by Dan Kitwood/Getty Images)
LONDON, ENGLAND - DECEMBER 05: Russell Brand seen at BBC Radio 2 on December 5, 2014 in London, England. (Photo by Neil Mockford/Alex Huckle/GC Images)
LONDON, ENGLAND - NOVEMBER 25: Russell Brand poses for photographs as he arrives to deliver The Reading Agency Lecture at The Institute of Education on November 25, 2014 in London, England. Russell Brand will deliver 'a manifesto on reading' which will be in part personal, sharing his own experience of books and reading while growing up in the UK. (Photo by Carl Court/Getty Images)