"Delta Bell Shepard is here," tweeted proud dad and Parenthood actor Dax on Friday.
"She doesn’t look like either Kristen or me, but we're gonna keep her anyway," he added jokingly.
Kristen and Dax have been together since 2007
Veronica Mars star Kristen pointed out that the arrival of another girl means Dax is decidedly outnumbered, tweeting: "The estrogen has now hit critical mass! Good luck, @daxshepard1. And welcome to the world DELTA BELL SHEPARD."
Kristen had hinted that she could be about to give birth earlier in the day on Friday when she joked that her emotional state must mean her baby was on its way
"Crying in the car to @joshgroban christmas songs," she wrote. "[This] has got to be an early sign of labor, right?"
Kristen and Dax, who tied the knot in 2013 after six years together, welcomed daughter Lincoln in March 2013.
The 34-year-old actress said their first-born had been taking a keen interest in her pregnancy, offering food to Kristen's bump.
"She's very interested in the shape of my belly," Kristen told Us Weekly. "Very interested in offering it water and crackers and giving it smooches.
"She could just be mimicking what we're doing," she added. "But I have a hunch she knows what's happening."