According to the birth certificate obtained by The Blast, the child was born at the Providence Saint John's Health Center in Santa Monica, California, on May 3.
Ennis is the couple's first child, who confirmed their pregnancy in January.
One of the sources had earlier told People magazine that Dunst, 35 amd Plemons, 29, welcomed "a healthy baby boy" over the last weekend and "everyone is doing great".
The couple started dating in 2016 after meeting on the sets of "Fargo" season two, where they played a married couple in the show.
They announced their engagement in January last year.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)