Parental supply of alcohol to adolescents is associated with increased risk of subsequent adolescent alcohol use and harms, so identifying factors associated with parents’ decision-making is critical. This study examined how parental supply is associated with attitudes toward adolescent alcohol use, perceived norms of parental supply, perceived behavioural control and perceived acceptable age to drink alcohol. A total of 1197 Australian parents with children aged 12–17 years completed an online cross-sectional survey assessing their parental supply behaviours, attitudes and perceptions in April 2022. Logistic regression was used to explore associations between attitudes, perceptions and parental supply of alcohol to their child. Forty-three percent of respondents nominated an acceptable age to drink a full drink of alcohol below 18 years, and 23% reported supplying a full drink of alcohol to their adolescent. Parents were more likely to report supplying a full drink of alcohol if they nominated an acceptable drinking age below 18 years (<16: adjusted odds ratio [AOR] = 14.75, 95% confidence interval [CI] = 8.23–26.42; 16–17: AOR = 5.68, 95% CI = 3.69–8.73), appraised alcohol as more beneficial (AOR = 1.31, 95% CI = 1.02–1.69) and less harmful (AOR = 0.49, 95% CI = 0.36–0.68) for adolescents, and perceived that parent friends (AOR = 2.91, 95% CI = 1.80–4.70) and other parents (AOR = 2.23, 95% CI = 1.37–3.62) supplied alcohol in unsupervised contexts. Perceived behavioural control was not associated with parental supply. These findings suggest there may be value in trialling interventions that target parents’ perceptions about the acceptable age to drink a full drink of alcohol, attitudes toward adolescent alcohol consumption, and perceived norms of parental supply to influence parents’ supply intentions.