Watkins, 39, shared the news with an adorable video, in which his and Ryder's heads are put on animated bodies.
"We are VERY excited to share our news with you," Watkins wrote on YouTube. "For a long time we have been wishing and our dream is about to come true."
— Ian H Watkins (@Ianhwatkins) December 15, 2015
The video features the animated story of the couple's relationship milestones, including buying a house and going on holiday.
Captions pop up, which read: "Together we've been on lots of adventures.
"But our biggest adventure is about to begin.
"For a long time we have been wishing and our wish is about to come true...
"We are going to be dads."
If that wasn't exciting enough, the animated heads had another little secret to unveil.
The couple will be dads to twins.
The animation ended with a picture of the ultrasound scan hanging over a fireplace with the caption: "The Ryder-Watkins twins – arriving soon!"
Watkins was overwhelmed with congratulations messages after tweeting a link to the video.
"Feeling the Love on here tonight. Thanks for all your messages," he wrote.
Feeling the Love on here tonight !! Thanks for all your messages ! 😍😍👶🏼👶🏼 #BabySteps— Ian H Watkins (@Ianhwatkins) December 15, 2015
A massive thank you to @carlkwan for making our animation 👶🏼👶🏼— Ian H Watkins (@Ianhwatkins) December 15, 2015
Watkins and Ryder have been open about their wish to become dads before.
Having previously spoken about plans to adopt, in 2012 the couple revealed their surrogate had suffered a miscarriage.
"We were thinking names, and we even planned our first Christmas as a family. We were both ready to be dads. I just feel empty and helpless. The only person who can help me is Craig."
Congratulations to the couple!