The pair welcomed an unnamed baby daughter on Sunday, who weighed in at 8lbs 9oz.
"[They are] thrilled that she is beautiful and healthy," a source told US Weekly.
The insider went on to say that Kimberly's parents Rod and his ex-wife Alana were similarly excited about the new arrival, adding: "Both mum and baby are doing really well.
"Rod and Alana were thrilled to become grandparents for the first time and delighted at the new addition to their family."
Del Toro's spokesperson confirmed that Stewart was expecting in April 2011, but added at the time that they are not a couple.
Watch Benicio del Toro in the trailer for The Wolfman below: