Oscar-nominated Sevigny shared a picture of the family on Instagram, showing her cradling the baby, named Vanja, while New York art gallery director Mackovic placed his head on her shoulder.
She wrote in the caption: “Welcome to the world Vanja Sevigny Mackovic Born May 2nd New York City.”
Sevigny, whose film roles include Boys Don’t Cry, Zodiac and Mr Nice, thanked staff at the hospital where she gave birth for their “bravery, perseverance and kindness”, adding: “Especially the nurses for being so gentle and patient. Blessings to all the other families giving birth during this time.”
Sevigny added the hashtag #ilovemyboys.
News of her pregnancy broke in January when she was spotted strolling through New York alongside Mackovic with her growing baby bump on show.
She has been dating Mackovic for over a year.
Actress Chloe Sevigny reveals her newborn son’s name
