A lifetime of exposure to ageism may be internalized in older adults, and these ageist beliefs that are directed inwards can have severe consequences. Unfortunately, research on reducing internalized ageism is scarce. To address this, I designed and implemented a six-week process-based intervention to reduce internalized ageism. This intervention utilized education, acceptance and commitment therapy, and attributional retraining. I evaluated the feasibility and effectiveness of this intervention in reducing internalized ageism, and the mechanisms through which it achieved this reduction. A total of 81 participants consented to the feasibility portion of the study, and 78 consented to the effectiveness portion of the study. Regarding feasibility, the program was evaluated overwhelmingly positively. Most participants rated each session and the overall program as very useful and indicated that it changed their perspectives on ageism. The program also maintained impressive engagement, with only three participants dropping out of the program, and the majority completing the between-session activities. Participants also attributed a wide range of novel behaviours to this intervention. Regarding effectiveness, participants’ internalized ageism was substantially reduced following the intervention and this was maintained at a two-month follow up; improvements were associated with large effect sizes. Additionally, this reduction of internalized ageism was mediated by increases in psychological flexibility, mindfulness, and perceived control. Altogether, this study provides a promising foundation from which to advance research on interventions that address the problem of internalized ageism – a problem that has severe consequences on the health and well-being of growing numbers of older adults in every country around the world.