| Honorific-Prefix: | The Honourable |
| William Postlethwaite | |
| Birth Date: | 17 September 1829 |
| Birth Place: | Broughton, Lancashire, England |
| Death Place: | Bootle, Cumberland, England |
| Order: | Member of Parliament for Geraldine |
| Term Start: | 1881 |
| Term End: | 1884 |
| Predecessor: | Edward Wakefield |
| Successor: | William Rolleston |
| Profession: | Farmer |
William Postlethwaite (17 September 1829 – 9 April 1908) was an English-born Member of Parliament representing the Canterbury region of New Zealand.
Postlethwaite was born in Broughton, Lancashire, the son of Robert and Agnes Postlethwaite. He was tutored by Branwell Brontë, brother to the Brontë sisters. He married Annie Camilla Brisco, daughter of Sir Robert Brisco, 3rd Baronet, in 1859.[1] He succeeded to his uncle's Oaks estate in Millom, Cumberland, but in an unusual move for landed gentry, moved to New Zealand in 1878.[2]
Postlethwaite represented the Geraldine electorate from 1881 to 1884, when he retired.[3] He was an independent politician.[4]
He then moved to California in 1891, living as a farmer in Lindsay,[5] but returned to England, where he died in 1908.[1]