James Martineau (; 21 April 1805 – 11 January 1900) was an English religious philosopher influential in the history of Unitarianism.
For 45 years he was Professor of Mental and Moral Philosophy and Political Economy in Manchester New College, the principal training college for British Unitarianism.
Many portraits of Martineau, including one painted by George Frederick Watts, are held at London’s National Portrait Gallery. In 2014, the gallery revealed that its patron, Catherine, Duchess of Cambridge, was related to Martineau. The Duchess’ great-great-grandfather, Francis Martineau Lupton, was Martineau’s grandnephew. The gallery also holds written correspondence between Martineau and Poet Laureate, Alfred, Lord Tennyson – who records that he “regarded Martineau as the mastermind of all the remarkable company with whom he engaged”. William Ewart Gladstone said of Martineau; “he is beyond question the greatest of living thinkers”.One of his children was the Pre-Raphaelite watercolourist Edith Martineau.