.Robert Gruntal Nathan (January 2, 1894 – May 25, 1985) was an American novelist and poet.
Nathan was born into a prominent New York family. He was educated in the United States and Switzerland and attended Harvard University for several years beginning in 1912. It was there that he began writing short fiction and poetry. However, he never graduated, choosing instead to drop out and take a job at an advertising firm to support his family (he married while a junior at Harvard). It was while working in 1919 that he wrote his first novel—the semi-autobiographical work Peter Kindred—which was a critical failure. But his luck soon changed during the 1920s, when he wrote seven more novels, including The Bishop's Wife, which was later made into a successful film starring Cary Grant, David Niven, and Loretta Young.

During the 1930s, his success continued with more works, including fictional pieces and poetry. In 1940, he wrote his most successful book, Portrait of Jennie, about a Depression-era artist and the woman he is painting, who is slipping through time. Portrait of Jennie is considered a modern masterpiece of fantasy fiction and was made into a film, starring Jennifer Jones and Joseph Cotten.

In January 1956 the author wrote, as well as narrated, an episode of the CBS Radio Workshop, called "A Pride of Carrots or Venus Well-Served."

Nathan's seventh wife was the British actress Anna Lee, to whom he was married from 1970 until his death. He came from a talented family — the activist Maud Nathan and author Annie Nathan Meyer were his aunts, and the poet Emma Lazarus and Supreme Court Justice Benjamin Cardozo his cousins.