The 32-year-old Olympic runner took to Twitter to announce the happy news, writing:
'Yeeeeezzzzzz!!! It's a boy!!! Buzzing to welcome the new man of the house!!!! Mum and baby doing well!!! #Excited'
The baby, whose name has not yet been released, is the first boy for the Farah family - they already have three-year-old twins, Amani and Aisha, and 10-year-old eldest daughter Rhianna, from Tania's previous relationship.
Mo and Tania, seen here at the launch of his waxwork, already have three daughters
Three days prior to the announcement, Tania, 27, shared a picture of her heavily-pregnant self exercising on the treadmill on her Instagram account.
'Trying to walk this baby out of me!' she told her followers in the caption.
Tania shared a picture of her none-month bump with her 4,800 Instagram followers
Mo and Tania married in Richmond, London, in 2010 and celebrated the birth of their twin daughters in 2012, just after his 10,000m win at the London Olympic games. Tania revealed at the time that her husband was 'bouncing off the walls' with happiness, and wanted to become a hands-on dad straight away - he was the first person to hold his children, and even cut the umbillical cord himself.
It was this enthusiastic attitude that led him to win Celebrity Dad of the Year in 2013.
'I train to be an athlete but there is no training to be a dad and I'm learning all the time,' he said upon receiving the accolade.
'This award shows I must doing something right, which is a real confidence boost!'
The couple then announced the pregnancy with baby number four in August 2015, and we're so pleased to hear that he's arrived safely.
Congratulations, Mo and Tania - we can't wait to see what name you choose for your bundle of joy!