Law, 33, shared the news on her Instagram page Thursday, alongside a photo from her hospital bed.
"So very happy to introduce you to Kinley," Law wrote in the caption. "7 pounds of baby perfection. Mama and Papa are doing well! Thanks for all the love and support during this pregnancy and to everyone who reached out to check in on us! I am already so in love with this bundle of squirming joy!!!"
The news was met with happiness from Law's fans, who congratulated her on Instagram.
"Oh my goodness. Beautiful pic. Many blessings to you and your hubby. God bless," one fan wrote.
"She is a beautiful gift from God to be handled with love. Congratulations," added another.
"Congratulations you will be the best mom," another fan wrote.
Kinley is Law and Andreen's first child. The couple tied the knot in January 2013 in Costa Rica, three years after they got engaged.
"Costa Rica is my favorite place in the world," Law told Life & Style after the wedding. "It was the ideal way to unwind from the stress and work of 2012 and recharge for 2013."
Law stunned in an Anna Maier Ulla-Maija Couture gown, which she picked after a three-year process to find her "perfect dress."
"One of my favorite moments was the fireworks display. My second favorite moment was when we all jumped into the pool, fully clothed," she said of the wedding.
Law plays the recurring role of Nyssa al Ghul on Arrow with her last appearance in the March episode "The Thanatos Guild." She has made a guest appearance in an episode of DC's Legends of Tomorrow and recently starred as Stephanie Bailey in Sacred Lies and the dirty cop Karen Beach in Sony/Crackle's The Oath.
Law also played Rebecca Lee on CBS' short-lived Training Day series, opposite the late Bill Paxton, and played Mira in Showtime's Spartacus series.
Law's other credits include The Resistance, CSI: Miami, Chosen, Uncle Buck and Darkness Rising.
Andreen, 44, is also an actor. His credits include Lemon Cove, Reaper Tales, Real Rob and The Resistance. He also appeared in episodes of CBS' Scorpion and A&E's Longmire.0comments
The Oath will return for Season 2 on Thursday, Feb. 21. The cast also includes Ryan Kwanten, Cory Hardrict, J.J. Soria, Leona Lewis, Kevin Connolly, Russell Wong and Christinia Milian.
Photo credit: Vincent Sandoval/Getty Images