Sir Isaac Alfred Isaacs (6 August 1855 – 11 February 1948) was an Australian lawyer, politician, and judge who served as the ninth Governor-General of Australia, in office from 1931 to 1936. He had previously served on the High Court of Australia from 1906 to 1931, including as Chief Justice from 1930.

Isaacs was born in Melbourne and grew up in Yackandandah and Beechworth (in country Victoria). He began working as a schoolteacher at the age of 15, and later moved to Melbourne to work as a clerk and studied law part-time at the University of Melbourne. Isaacs was admitted to the bar in 1880, and soon became one of Melbourne's best-known barristers. He was elected to the Victorian Legislative Assembly in 1892, and subsequently served as Solicitor-General under James Patterson, and Attorney-General under George Turner and Alexander Peacock.

Isaacs entered the new federal parliament at the 1901 election, representing the Protectionist Party. He became Attorney-General of Australia in 1905, under Alfred Deakin, but the following year left politics in order to become a justice of the High Court. Isaacs was often in the minority in his early years on the court, particularly with regard to federalism, where he advocated the supremacy of the Commonwealth Government. The balance of the court eventually shifted, and he famously authored the majority opinion in the Engineers case of 1920, which abolished the reserved powers doctrine and fully established the paramountcy of Commonwealth law.

In 1930, Prime Minister James Scullin appointed Isaacs as Chief Justice, in succession to Sir Adrian Knox. Later that year, Scullin nominated Isaacs as his preferred choice for governor-general. The selection of an Australian (rather than the usual British aristocrat) was unprecedented and highly controversial. King George V was opposed to the idea but eventually consented, and Isaacs took office in January 1931 as the first Australian-born holder of the office. He was the first governor-general to live full-time at Yarralumla, and throughout his five-year term was popular among the public for his frugality during the Depression. Isaacs was Australia's first Jewish High Court Justice, the first Jewish Chief Justice of Australia and also the first Jewish Governor-General of Australia, and in retirement became known for his strident anti-Zionism.