The Family Law Act 1975 (Cth) is an Act of the Parliament of Australia. It has 15 parts and is the primary piece of legislation dealing with divorce, parenting arrangements between separated parents (whether married or not), property separation, and financial maintenance involving children or divorced or separated de facto partners: in Australia. It also covers family violence. It came into effect on 5 January 1976, repealing the Matrimonial Causes Act 1961, which had been largely based on fault. On the first day of its enactment, 200 applications for divorce were filed in the Melbourne registry office of the Family Court of Australia, and 80 were filed in Adelaide, while only 32 were filed in Sydney.

Though the Commonwealth had the power since federation in 1901 to make laws affecting divorce and related matters such as custody and maintenance, it did not enact such national uniform laws until 1961, when the Matrimonial Causes Act 1959 came into operation. The Act continued the fault-based system operating under state authority. Under the Commonwealth law a spouse had to establish one of the 14 grounds for divorce set out in the Act, including adultery, desertion, cruelty, habitual drunkenness, imprisonment and insanity. In reality, the system was very expensive and humiliating for the spouses, necessitating appointment of barristers, often private detectives, collection of evidence, obtaining witness statements, photographs and hotel receipts, etc. Failure to prove a spouse's guilt or wrongdoing would result in a judge refusing to grant a divorce. The Matrimonial Causes Act 1959 was replaced by no-fault divorce system of the Family Law Act 1975.

The Act was first introduced as a Bill on 13 December 1973. Before the Bill became law, it lapsed and was reintroduced on 3 April 1974 with substantial changes. A third reintroduction was made after the Bill lapsed a second time, with the final reintroduction made on 1 August 1974 with additional changes. The Act was contentions due to its reform of divorce laws. The legislation meant divorce could be obtained with one requisite being 12 months separation. A Gallup Poll taken during negotiation of the Bill showed 64% of men and 62% of women respectively supporting these changes.

The Act was enacted in 1975 by the Australian government, led by then Prime Minister Gough Whitlam. One of the main innovations was the introduction of no-fault divorce. Couples no longer needed to show grounds for divorce, but instead, just that their relationship had suffered an irreconcilable breakdown.

Due to the division of power between the Commonwealth and the Australian states under the Australian Constitution, the Act initially could deal with children born or adopted only within a marriage, it was not until later years that the Act dealt with matters relating to ex-nuptial children. However, the states referred these powers to the Commonwealth and, until the 2006 amendments to the law, were all located under Chapter VII of the Act. For limitations on recognition of de facto couples inside and outside of Australia see Section 51(xxxvii) of the Constitution of Australia.

For Australian Divorce Law, see Australian family law

The Act revolutionised the divorce law of Australia by replacing the previous fault grounds with the single ground of irretrievable breakdown, established by separation and living apart for a period of twelve months. It also reduced the time for a decree nisi for a divorce to take effect from three months to one month.

Amendments in 2004 abolished the provisions dealing with "decrees nisi" or "decrees absolute" and changed the term dissolution of marriage to divorce. The twelve-month separation requirements remained and the one-month waiting period for a divorce order to take effect remained.