There is preliminary support for internet-delivered cognitive behaviour therapy (iCBT) as a way of improving access to treatment among older adults with anxiety. The aim of this randomized controlled trial (RCT) was to examine the efficacy, long-term outcomes, and cost-effectiveness of an iCBT program for adults over 60 years of age with anxiety. Successful applicants were randomly allocated to either the treatment group (n = 35) or the waitlist control group (n = 37). The online treatment course was delivered over 8 weeks and provided with brief weekly contact with a clinical psychologist via telephone or secure email. Eighty-four percent of participants completed the iCBT course within the 8 weeks and 90% provided data at posttreatment. Significantly lower scores on measures of anxiety (Cohen’s d = 1.43; 95% CI: 0.89 – 1.93) and depression (Cohen’s d = 1.79; 95% CI: 1.21 – 2.32) were found among the treatment group compared to the control group at posttreatment. These lower scores were maintained at 3-month and 12-month follow-up and the treatment group rated the iCBT treatment as acceptable. The treatment group had slightly higher costs ($92.2; 95% CI: $38.7 to $149.2) and Quality-Adjusted Life-Years (QALYs = 0.010; 95% CI: 0.003 to 0.018) than the control group at posttreatment and the intervention was found to have a greater than 95% probability of being cost-effective. The results support iCBT as an efficacious and cost-effective treatment option for older adults with symptoms of anxiety. Trial Registration: Australian and New Zealand Clinical Trials Registry: ACTRN12611000929909; https://www.anzctr.org.au/Trial/Registration/TrialReview.aspx?ACTRN=12611000929909.