Sir John William Downer, KCMG, KC (6 July 1843 – 2 August 1915) was an Australian politician who served two terms as Premier of South Australia, from 1885 to 1887 and again from 1892 to 1893. He later entered federal politics and served as a Senator for South Australia from 1901 to 1903. He was the first of four Australian politicians from the Downer family dynasty.

Downer was born in Adelaide on 6 July 1843. He was one of four sons born to Jane (née Field) and Henry Downer; his father was a tailor. His parents had immigrated from England to South Australia in 1838, among the first waves of British immigrants brought out by the South Australian Company.

Downer began his education at a private academy in Adelaide. He went on to attend St Peter's College, Adelaide, on a scholarship. In 1862 he took first prize in the statewide examinations. Downer followed his older brother Henry Edward Downer into the legal profession, serving his articles of clerkship with Henry and with James Boucaut. He was called to the bar in 1867. He subsequently went into practice with his younger brother George Downer, with their firm G. & J. Downer becoming "one of the leading Adelaide law firms for the next sixty years".

Downer was elected unopposed to the South Australian House of Assembly at the 1878 general election, standing in the two-member seat of Barossa. He continued to represent Barossa until his resignation to enter federal parliament in 1901. His electorate spanned from Gawler across the Barossa Valley to the River Murray; his primary support base was in Gawler.

In June 1881, Downer was appointed attorney-general in the ministry formed by John Cox Bray, serving until the government's defeat after the 1884 election. He introduced a number of legal reforms, notably the Married Women's Property Act 1883 which abolished the law of coverture and allowed married woman to own property, enter into contracts and take part in lawsuits in their own right. He was an early advocate of universal women's suffrage and opposed attempts to include a property qualification, although he did not support the right of women to stand for parliament. He also succeeded in carrying bills allowing accused persons to give evidence on oath, and amending the insolvency and marriage acts.

On 16 June 1885, Downer himself became Premier for the first time, as well as being Attorney-General once again. While Premier, Downer oversaw the construction of the first train line from Adelaide to Melbourne. He also made significant contributions to establishing irrigation settlements along the Murray River. Although this ministry lasted two years and passed a fair amount of legislation, it was often in difficulties, and in June 1886 had to be reconstructed.

At the Colonial Conference held in London during 1887, Downer represented South Australia, but during his return journey to Australia his government was defeated. This ministry was responsible for a tariff imposing increased protective duties. Downer was not in office again for several years, but in October 1892 again became Premier, taking also the portfolio of Chief Secretary. In May 1893 he exchanged this for the position of Treasurer of South Australia, but was ousted at the 1893 election by liberal Protectionist Kingston with the support of the new Labor Party led by John McPherson. Downer remarked of this party: 'They are very clever fellows. I have great respect for the way they use either side for their purposes with absolute impartiality'. For most of the period until 1899 Downer led the Opposition.

Downer was a strong federalist and had represented South Australia at the 1883 and 1891 conventions. At the latter meeting, he took an important part in protecting the interests of the smaller states, and was a member of the constitutional committee. He was elected one of the 10 representatives of South Australia at the Australasian Federal Convention of 1897-8, and sat, again, on the constitutional committee. He was the most senior and significant representative of the conservative portion of the Convention's ideological spectrum. In this period he formed a close personal bond with several leading Federationists. Robert Garran, an eminence grise of the Federalist cause, was the best man at Downer's wedding in 1898. Richard O'Connor and Edmund Barton were among the attendees. The bride and groom had first met at Barton's house, and in these years Barton would stay at Downer's house when in Adelaide.