The Season 17 The Bachelor couple announced the big news of Catherine's second pregnancy on Friday, November 3 via social media.
"It's happening!" Sean captioned a video on Instagram.
In the video, Samuel is sitting on the couch, looking absolutely adorable, when Sean can be heard asking behind the camera, "Hey Samuel, can you say baby?"
Samuel then says, "baba" and points to Catherine's belly as she giggles and exclaims, "Baby!"
Catherine captioned a similar video, "Baba numba two," on her own Instagram account.
In addition to a picture of Samuel sitting in her lap, a video shows Catherine asking her 16-month-old son, "Samuel, what's this?" to which he replies, "Baba" while lovingly stroking his mother's pregnant stomach.
Sean and Catherine just celebrated Samuel's birthday in June 2017, gifting him with balloons and a giant teddy bear.
Just last month, the reality TV stars told Us Weekly they were ready to have another baby so their children would be roughly two years apart in age.
"The nursery is pretty set in case baby number two comes around any time soon," Catherine told Us at the time.
In September, Sean told People that he was fully expecting to expand his family in the near future, as he said the possibility of a second child was "not off in the distance because Catherine wants a big family -- she's convinced she wants several more kids."
"I'm 33 going on 34, so we better get crackin' sooner or later!" Sean quipped to the magazine. "I would say that hopefully in 2018 we have another one."
Sean and Catherine got engaged on The Bachelor's finale in 2013. After Sean competed on Season 16 of Dancing with the Stars that year, the couple tied the knot in a live ABC television special that aired in January 2014.
They both subsequently appeared on Celebrity Wife Swap and Marriage Boot Camp before Samuel was born last year.
The Bachelor is heading into its 22nd season with new star Arie Luyendyk Jr., whom Sean met and became friends with while competing for Emily Maynard's heart on The Bachelorette's eighth season. (Arie finished as the runner-up, while Sean placed third).
Sean is Arie's biggest fan, as he recently told Us that the 36-year-old professional race car driver turned real estate agent has a great personality in that he's "quirky," "nerdy" and "endearing."