The Pretty Little Liars star revealed the exciting news to Instagram on Thursday.
"She's here and she's perfect", the 35-year-old actress wrote alongside a love heart emoji.
Tammin is yet to share a photo of her new bub or reveal her name.
Earlier this week Tammin revealed the emotional struggle she's gone through after getting pregnant following two miscarriages.
Speaking on Dr Berlin's Informed Pregnancy podcast, she said her past heartbreak made it difficult for her to bond with her second child.
"I didn't connect to this baby for so long. I think my husband did too. I was like, 'Are you connected to this baby?' And he was like, 'I'm trying'," Tammin said.
"And then it's almost like when the baby turned, I felt like we were communicating. Until she could survive on her own [was] when I started to go, 'This is real'."
Tammin explained that although her miscarriages were "traumatic", she wanted to share her story in order to help other women.
"It took me a long time to share it. But if I can make one person feel less alone…Because it happens so much, I think it's one in four [women]."
The couple already shares 4-year-old daughter Phoenix.