The actor's daughter, Drena De Niro, shared the news of her son Leo's death on Sunday.
In a statement issued to ET on Monday, the actor expressed profound sadness over the loss. De Niro's daughter, Drena De Niro, revealed late Sunday evening that her son, whom the family called "Leo," had died.
"I'm deeply distressed by the passing of my beloved grandson Leo," the Raging Bull star said in a statement via his rep. "We're greatly appreciative of the condolences from everyone. We ask that we please be given privacy to grieve our loss of Leo."
In an emotional Instagram post, Drena, 51, paid tribute to her late son. Leandro was reportedly 19 at the time of his death.
"My beautiful sweet angel . I have loved you beyond words or description from the moment I felt you in my belly .You have been my joy my heart and all that was ever pure and real in my life . I wish I was with you right now. I wish I was with you . I don’t know how to live without you but I’ll try to go on and spread the love and light that you so made me feel in getting to be your mama," she wrote. "You were so deeply loved and appreciated and I wish that love alone could have saved you 😞I’m so sorry my baby , I’m so sorry @carlosmare 😢💔. Rest in Peace and Eternal Paradise my darling boy 🙏🏽💔💔💔💔💔🕊️🕊️🕊️🕊️"
Drena later told ET in a statement, “It is with immeasurable shock and sadness that we say goodbye to our beloved son Leo. We thank you for the outpouring of love and support and ask that we are given privacy at this time to process this inconsolable grief."
Drena did not share any additional details about Leandro's death. The boy's father, artist Carlos Mare, also seemingly addressed the loss on his Instagram by posting a simple black square on his feed.
Carlos commented on the post, "😢 My dear Drena… words aren’t enough to express the joy he gave us or the loss we now endure with our families and friends. He is Godschild now. 🙏🏼 On this full moon his spirit luminates what could not be seen in the dark. You can’t spell LOVE without LEO 👼 💔 💔"
Lenny Kravitz was also among those offering heartfelt condolences.
"Dearest Drena, sending my deepest condolences to you and the family. This is all beyond words at the moment. I love you," the singer commented.
Naomi Campbell added, "Drena heartbroken for you , such a Duo , i can’t imagine how you must feel , May the lord cover you and hold you in his arms ,at this very sad time and loss .. I love you , always here for you 🕊️💔🙏🏾."
Andy Cohen also reached out, commenting: "Drena I am so sad to hear about beautiful Leo. Sending you all my love ❤️."
Drena and Leandro previously appeared together in Bradley Cooper's 2018 film, A Star Is Born. Drena played Paulette Stone -- the wife of Dave Chappelle's character, George "Noodles" Stone -- while Leandro played their son, Leo Stone.
The mother-son pair also share acting credits in 2005's The Collection and 2018's Cabaret Maxime.
De Niro shares his adult children, Drena, 51, and Raphael, 46, with ex-wife Diahnne Abbott, as well as 27-year-old twin sons, Aaron and Julian, with former girlfriend Toukie Smith. He also shares son Elliot, 25, and daughter Helen, 11, with ex-wife Grace Hightower.