"It's something they've talked about a lot but never felt ready to do until now," the friend says of the couple, who hooked up in 2014 on the set of their FX thriller, The Americans, and share son Sam, 4.
While the A Beautiful Day in the Neighborhood actor, 46, already has several TV projects lined up, Russell, 45 — who's also mom to River, 13, and Willa, 9, with ex-husband Shane Deary — "is organizing her schedule so she can enjoy her pregnancy," explains the friend.
"Keri and Matthew are more in love than ever. They've never felt the need to get married because they've built a wonderful life together," continues the friend, "and now they can't wait to grow their family."
"Matthew and Keri have a blast being parents," dishes the pal of the longtime couple.
Rhys and Russell were known for their sizzling on-screen chemistry even before they started dating. Co-starring on the The Americans, the two shared several steamy kissing scenes that drove fans wild.
"We've all done a million of them in this crazy life we sign up for," Rhys told PEOPLE of perfecting the intimate on-screen moments. "The first one is always the hardest, and then you realize there's a very perfunctory element to it where cameramen shout, 'Hand up more! Down with your elbow! Lift your leg up higher!'"
Rhys' connection with the Felicity alum actually goes way back, as he revealed to Andy Cohen on Watch What Happens Live in 2017. He said he fist met his future love in a parking lot after a "kickball party."
"We actually met a very long, long time ago I very drunkenly asked her for her number when she was a young, single slip of a thing," he recalled. "So I sort of knew then, when I was 26."