Peter Martin Duncan (20 April 1824 - 28 May 1891) was an English palaeontologist.
Duncan was born in Twickenham, and was educated partly at the local grammar school and partly in Switzerland. Having entered the medical department of King's College London in 1842, he obtained the degree of M.B.(Lond.) in 1846, and then acted for a short time as assistant to a doctor at Rochester. Subsequently he practised at Colchester (1848 - 1860), and during this period he served for a year as mayor of the city.
Returning to London in 1860 he practised for a few years at Blackheath, and then gave his time entirely to scientific research, first in botany, and later in geology and palaeontology. His attention was directed especially to fossil corals, and in 1863 he contributed to the Geological Society of London the first of a series of papers on the fossil corals of the West Indies in which he not only described the species, but discussed their bearings on the physical geography of the Tertiary period. Corals from various parts of the world and from different geological formations were subsequently dealt with by Duncan, and he came to be regarded as a leading authority on these fossils. He prepared also for the Palaeontographical Society (1866 - 1872) an important work on British fossil corals, as a supplement to the monograph by Henri Milne-Edwards and Jules Haime. He was elected Fellow of the Royal Society in 1868. In 1870 he was chosen professor of geology at King's College. He was president of the Geological Society (1876-1877), and in 1881 was awarded the Wollaston medal. In addition to papers on fossil corals, he dealt with some of the living forms, also with the Echinoidea and other groups, recent and fossil. He edited the six volumes of Cassell's Natural History (1877). He died at Gunnersbury.