Ronald John Withnall (9 December 1914 – 16 September 1990) was an Australian politician and lawyer.
Withnall was born in Townsville, Queensland, but attended school at Canterbury High School in Sydney. He studied law at the University of Sydney and later moved to the Northern Territory. He served as the Crown Law Officer for the Territory from 1954 to 1968, when he was elected to the Northern Territory Legislative Council, the forerunner to the current Legislative Assembly. In the same year, Withnall also became the inaugural President of the Law Society of the Northern Territory.
As a Member of the Legislative Council, Withnall made a name for himself as a fierce advocate of self-determination, and took part in the campaign which ultimately led to the creation of the Assembly in 1974. He ran as an independent in the seat of Port Darwin in the first elections, and ultimately became one of only two non-Country Liberal Party members (the other being fellow independent Dawn Lawrie) in the first Assembly. However, he was to only serve one term in the Assembly, and was replaced by the Country Liberal Party's Tom Harris.
Withnall died in late 1990, and several condolence motions were presented in the Assembly upon his death. Shane Stone, in his introductory speech upon his election to the Port Darwin electorate in 1990, referred to Withnall as a "fearless and uncompromising advocate of self-government and statehood for the Territory". In 1994, then-Chief Minister Marshall Perron also praised Withnall's role in the Territory's achievement of self-governance. A 1976 interview with Withnall is on file at the National Library of Australia.