Isaac Roberts FRS (27 January 1829 – 17 July 1904) was a Welsh engineer and businessman best known for his work as an amateur astronomer, pioneering the field of astrophotography of nebulae. He was a member of the Liverpool Astronomical Society in England and was a fellow of the Royal Geological Society. Roberts was also awarded the Gold Medal of the Royal Astronomical Society in 1895.

Roberts was born at Groes Bach, Henllan, Denbighshire, to William Roberts, a farmer, and his wife Catherine Roberts, née Williams, in January 1829. Although he spent some years of his childhood there, he later moved to Liverpool. There, he became an apprentice to John Johnson & Son (which later became Johnson and Robinson), a firm of mechanical engineers, for 7 years beginning on 12 November 1844. He became a partner in 1847, and supplemented his job with night school. When Peter Robinson died in 1855, Roberts was made manager of the firm. When the other partner, John Johnson, died, Roberts was in charge of the contracts and affairs of the firm. Roberts began working as a builder in 1859, and was joined by Peter Robinson's son, J. J. Robinson, in 1862. He was very successful, and became known as one of the best engineers in the region.

Roberts was an International Member of the American Philosophical Society.

Roberts married his first wife, Ellen Anne (Minnie) Cartmel (1852–1901), daughter of Anthony Cartmel, a boat builder, and his wife Ann, on 22 July 1875, at St Thomas, Lydiate, then in Lancashire. Ellen Anne Roberts was buried in Liverpool on 30 March 1901. Her address was listed as Kennessee House, Maghull. Roberts married Dr Dorothea Klumpke (1861–1942) in October 1901. They had met in 1896 at Vadsø in Norway while both were there to observe the total eclipse of the sun. Klumpke worked at the Paris Observatory.

He became agnostic in his religious views.

Roberts died suddenly in Crowborough, Sussex, England in 1904, aged 75, widowing his second wife Dorethea Klumpke. He was cremated soon after his death, and his ashes lay in Crowborough for about five years before he was reburied in Flaybrick Hill Cemetery, in Birkenhead. Roberts was patriotic to his home land of Wales, and continued to use the Welsh language throughout his life. He left a substantial amount of money to Cardiff University, Bangor University, and University of Liverpool. His epitaph reads:

Through a donation of his wife Dorothea in honor of her late husband, the Société astronomique de France (the French Astronomical Society) established the Prix Dorothea Klumpke-Isaac Roberts for the encouragement of the study of the wide and diffuse nebulae of William Herschel, the obscure objects of Barnard, or the cosmic clouds of R.P. Hagen. This biennial prize was first given in 1931 and continues today.

The crater Roberts on the far side of the Moon was named to jointly honour Isaac Roberts and the South African astronomer Alexander William Roberts.