It looks like he was not adopted. He was possibly born in, but does not look likely. His parents were artists under the tutelage of the modernist/abstract expressionist Hans Hoffman. His parents divorced when he was a boy, his father announcing that he was gay. Here is the pertinent Wikipedia info.
Robert Anthony De Niro Jr.[5] was born on August 17, 1943, in the Greenwich Village area of Manhattan, New York, the son of painters Virginia Admiral and Robert De Niro Sr.[6] He is of Irish and Italian descent on his father's side,[7] while his mother had Dutch, English, French, and German ancestry.[8] De Niro's parents, who had met at the painting classes of Hans Hofmann in Provincetown, Massachusetts, divorced when he was two years old after his father announced that he was gay.[9] De Niro was raised by his mother in the Greenwich Village and Little Italy areas of Manhattan. His father lived within walking distance and De Niro spent much time with him as he grew up.[10] His mother was raised Presbyterian but became an atheist as an adult, while his father had been a lapsed Catholic since the age of 12.[9][11] Against his parents' wishes, his grandparents had him secretly baptized into the Catholic Church while he was staying with them during his parents' divorce.[9][11]
https:// en. wikipedia.org/wiki/Robert_De_Niro