On June 3, 1874, Gilder married a daughter of Commodore George Coleman De Kay, Helena de Kay Gilder (1846–1916). Gilder met her in May 1872 while she was visiting the offices of Scribner's Monthly, where he was at the time working as an editor. About a year and a half later, in February 1874, Helena and Richard became engaged. They are known to have kept a lengthy correspondence with each other via letter over the course of their marriage. Helena de Kay Gilder is also known to be the subject of love poems written by Richard Watson Gilder, and they partnered together on some of his books, with her working as the illustrator, such as in Two Worlds and Other Poems (1891). She was a talented painter and a founder of the Art Students League and Society of American Artists. She also modeled for, and was an unrequited love of, the painter Winslow Homer. Gilder and de Kay were the models for the characters Thomas and Augusta Hudson in Wallace Stegner's Pulitzer-prize winning novel, Angle of Repose. Their son, Rodman de Kay Gilder (1877–1953), became an author and married Louise Comfort Tiffany, a daughter of Louis Comfort Tiffany. Their daughter, Rosamond Gilder, was a notable theater critic. A celebrated plaster sculpture of the family by Augustus Saint-Gaudens is owned by the Metropolitan Museum of Art. The Berkshires summer home of Helena de Kay and Richard Watson Gilder is being turned into a museum.