The 44-year-old actor appeared on "The Late Late Show with James Corden" on Wednesday, January 23, when he announced the exciting news while reflecting on life in his 40s.
"This is something I haven't even mentioned to anyone, if we could just keep it between the three of us and you all," Ferguson quipped, referencing himself, James Corden, fellow guest Charlie Hunnam, and the audience.
"I'm actually expecting a baby in July with my husband. But shh, don't tell anyone. Let's keep it between us," he said, adding, "I'm very excited. I'm 44 now, I'm like, let's get this show going, tick tock."
When James asked if the new arrival was a boy or a girl, the actor replied, "A human!"
The couple tied the knot in 2013, but have been discussing becoming dads for a while.
"Justin and I are very excited to come dads in the near future," he told Entertainment Tonight in 2017.
"No announcement yet, nothing has happened, but yeah, we are excited. We have been together for four years and we always said, 'Let's give ourselves five years just to be married,' and that's coming up, five years."