The 57-year-old actor welcomed his third son, Raphael, in June 2015, but the family are already expecting another boy to add to their growing brood.
Wife Hilaria, 32, took to Instagram to confirm the happy news, telling her 136,000 followers;'Ireland, Carmen, Rafael, Alec, and I are excited to share with you that we are going to have another addition to our family. A little boy coming this fall.'
In the three years that the couple have been married they have welcomed a daughter Carmen, two, and son Rafael.
Alec's fourth child is his eldest daughter Ireland, 20, from his first marriage to actress Kim Basinger.
The Academy Award winner himself comes from a family of four brothers, of which he is the eldest, so perhaps he is trying to replicate that with his own!
Alec and Hilaria, a yoga teacher, starting dating in August 2011 after Alec spotted her in a restaurant and handed her his business card and mobile number.
They quickly moved in together in the upmarket New York neighbourhood of Greenwich Village.
The Golden Globe winner proposed to his new girlfriend in April 2012 and they married in June of the same year.
Alec told Hello! magazine at the time; 'Marriage was made for you to make a commitment to someone who is worth it, and I met someone who is worth it.'
In 1990 Alec met his first wife-to-be Kim Basinger while they were acting opposite each other in The Marrying Man. The pair tied the knot in 1993 and filed for divorce in 2001.
Alec's daughter Ireland
Fans have congratulated the couple, with one commenting; 'Oh my goodness how exciting! You are such a great mum!'
Another added; 'This is wonderful news!! Congratulations to you and your beautiful family!! We all look forward to more baby posts!'