John Postle Heseltine was a painter and art collector who became a trustee of the National Gallery, London.

Heseltine was born on 6 January 1843 in Dilham, Norfolk. He was a son of Mary and Edward Heseltine. His brother was Rev. Ernest Heseltine, M.A. of Sandringham, who assisted the officiation of his eldest daughter Dorothy's marriage to Viscount Cantelupe in 1890. Through his brother Ernest, he was uncle to civil servant Michael Heseltine, the Registrar of the General Medical Council between 1933 and 1951.

In 1859, at age 16, he was sent to Hanover to learn German. While there, he was introduced to etching on copper by Major van Usslar-Gleichen. Heseltine quickly became a skilled draughtsman and engraver and exhibited his first etching, Hastings, at the Royal Academy in 1869. He joined the Etching Club in 1877, and was a founding member of the Society of Painter-Etchers in 1880.

Heseltine was a stockbroker and senior partner in the family firm, Heseltine, Powell & Co., which was founded by his father and Charles W. Marten in 1848 as Marten & Heseltine, and dealt particularly in American railroad bonds and shares. After his father retired, Thomas Wilde Powell was senior partner and Heseltine was junior partner. They supported bond issues for the New York and Erie Rail Road, Baltimore and Ohio Railroad (1873) and Pennsylvania Railroad (1876). Many years after his death, the name of the company changed to Heseltine, Moss & Co. in 1977 and the business became part of Brown Shipley Ltd. in 1987.

From 1893 until his death in 1929, Heseltine was a trustee of the National Gallery and advised on the purchase of paintings, particularly works from the Dutch and Flemish schools. Beginning in 1905 and lasting for the eighteen month period between Sir Edward Poynter's retirement as director and the appointment of Charles Holroyd, he shared responsibility for running the Gallery with Lord Carlisle, a fellow trustee.

"Heseltine was a keen collector of oil paintings, drawings and watercolours of the English and Continental schools. Among the old master drawings were specimens by Rembrandt, Rubens, Raphael, Michelangelo, Fra Bartolomeo, Holbein, Dürer, Constable, Watteau and Boucher." In 1912, after fifty years of collecting, he sold his collection of over 600 old master drawings to the London dealer Colnaghi & Obach for a price near $1,000,000. Thirty-two of his Rembrandt drawings sold the following May at what was then a high average of over $3,750 per drawing. In his collection, Heseltine also amassed a substantial collection of etchings by the Norwich School of painters.

Heseltine died at his home in Eaton Square on 2 March 1929 and his widow died on 11 January 1935.