The Without a Trace star and longtime boyfriend Adam Kaufman welcomed their first child together Dec. 23, son Jackson Phillip Kaufman, the actress' rep confirms to E! News.
The little one was born in Los Angeles, weighing 7 pounds, 4 ounces, according to publicist Jennifer Shoucair.
"This was the best Christmas present Poppy and Adam could have hoped for," she said. "Both she and the baby are doing wonderfully."
The thespian couple, who met more than three years ago on the Mexico set of the indie film Between, currently cohabitate in L.A.
To explain the burgeoning belly bump, Montgomery's pregnancy was written into Without a Trace's sixth-season plot line, with her due date to coincide with May sweeps.
But thanks to the ongoing writers' strike, FBI agent Samantha Spade is expecting after a one-night stand with a bartender (played by Kaufman, incidentally)—and audiences are still expecting a resolution.
Production on the CBS crime procedural shut down in November and it's anybody's guess when the current television season will get back on track.
Regardless, Montgomery's pending motherhood status was five months ahead of the show, according to tvguide.com, which first reported news of the birth. So the 35-year-old actress would have been sporting a prosthetic tummy in the future anyway.
Before joining the small-screen Missing Persons Unit in 2002, Montgomery made appearances in such dramatic fare as NYPD Blue, Party of Five and Silk Stalkings.
The native Australian also starred in the short-lived WB series Glory Days and in the 2005 TV movies Murder in the Hamptons and Snow Wonder.
Kaufman's credits include guest spots on Veronica Mars, Law & Order, Mad Men, CSI: NY, Monk and, playing a bartender who's been a little slow in signing those papers relinquishing his custodial rights, Without a Trace.