The 'Gossip Girl' actress and husband Adam Brody reportedly welcomed their baby into the world last week, a source told Us Weekly magazine.
The couple, who did not publicly announce their pregnancy, have not revealed the gender or the name of their new arrival.
It was revealed in May that the pair were expecting their first child.
The couple tied the knot in a secret ceremony in January 2014 after first meeting on the set of their 2011 film 'The Oranges'.
A source said at the time: "Leighton and Adam got married in Northern California by the ocean a couple of weeks ago. It was a very small and intimate wedding with close family and friends. It was beautiful."
The 29-year-old star first hinted she and her 35-year-old beau were trying for a baby during a recent magazine interview.
She said: "Most people I know are starting to get married. It'll be kids next. I like the grown-up stuff. I like having a house. I've got dogs.
"I started ['Gossip Girl'] when I was so young. And once it was over, I wanted to do things that were exciting and challenging and more me, more my adult taste."
Meanwhile, Adam previously joked he is hoping to have a very big family with Leighton.
Asked how many children they were planning on having, he said: "I say twelve. And I decide."