Sir William Agnew, 1st Baronet (20 October 1825 – 31 October 1910) was an English politician and art dealer. Thomas Agnew & Sons, his London art business in Mayfair flourished as one of the leading art dealerships in London from 1860, until it closed in April 2013, still with the Agnew family involved, and still known as "Agnew's Gallery", or more informally "Agnew's".
Agnew was a Liberal Member of Parliament, first for South East Lancashire between 1880 and 1885 and later for Stretford from 1885 to 1886. He was created a baronet, of Great Stanhope Street, London, in 1895.
He bought the Rougham estates in Suffolk, England, in 1904.
He was the son of Thomas Agnew (1794–1871) and his wife Jane Garnet Lockett.
On 25 March 1851, he married Mary Kenworthy (before 1836 – 2 September 1892), a daughter of George Pixton Kenworthy. Their children were:
Agnew's present-day descendants include Sir John Keith Agnew, 6th Baronet, of Rougham, and John Stuart Agnew, a parliamentary candidate of the UK Independence Party.