Background In the last decade, alcohol consumption among middle-aged women (40-65 years old) in Australia increased, despite declines in overall population consumption. Web-based, brief interventions are promising for reducing alcohol consumption, with efficacy shown in a wide range of populations. However, no published interventions have been designed specifically for and tested with middle-aged women. Objective This study aims to design and implement a web-based intervention intended to reduce alcohol consumption among middle-aged women. Methods The study is a 3-arm randomized controlled trial with a web-based intervention plus ecological momentary assessment (EMA) group compared to an EMA-only and a pre-post only control group. The study is aimed at middle-aged women, defined as women aged between 40 and 65 years, who consume alcohol at least weekly or who have consumed 4 or more drinks on 1 occasion in the last month. The intervention aims to reduce alcohol consumption through 4 modules that provide information on the health impacts of alcohol, mindfulness, social influences, and alcohol marketing. Intervention participants will also fill out biweekly EMA assessments. The comparators are EMA-only and pre-post control only. The primary outcome is alcohol consumption at 8 weeks compared between groups. Secondary outcomes are awareness of alcohol-related harms, readiness to change alcohol consumption, health status, mental health, and social support. Results Ethics approval for this project was received on September 11, 2019. The trial was registered on August 14, 2020. Recruitment has commenced, and the expected results will be available in 2022. Conclusions This web-based intervention aims to reduce alcohol consumption among middle-aged women, a currently understudied cohort in alcohol research. Trial Registration Australia New Zealand Clinical Trials Registry (ANZCTR) ACTRN12620000814976; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=12620000814976 International Registered Report Identifier (IRRID) DERR1-10.2196/34842