Stringfield, 41, gave birth to the baby in December.
"They've had a baby, and how they kept it a secret nobody knows," a source told the Post.
"Steve's very private," the insider said. "They are thrilled. They worked hard to have the baby."
The child's gender and name were not reported.
Stringfield and Martin married in 2007. They live in Los Angeles.
Justin Bieber crashes Drake Bell's album release party
Members of Congress to keep receiving porn magazine