Background: Postsecondary student suicide is one of Japan's most severe public health problems. Gatekeeper training (GKT) programs are a generally recommended suicide prevention intervention in Japan. For suicide countermeasures, an online program tailored to students may enhance self-efficacy as a gatekeeper. Objective: This study aims to describe a research protocol to investigate the effect of a newly developed internet-delivered online peer GKT program to improve postsecondary student self-efficacy as gatekeepers for suicide countermeasures in Japan. Methods: This study is a 2-arm, parallel, randomized controlled trial with a 1:1 (intervention: waiting list) allocation. Participants (n=320) will be recruited, and those who meet the inclusion criteria will be randomly allocated to the intervention or waiting list control group. An approximately 85-minute, 6-section, internet-based gatekeeper program for postsecondary students has been developed that includes videos to help participants acquire skills as gatekeepers. The intervention group will complete the program within 10 days. The primary outcome, self-efficacy as a gatekeeper, is measured using the Gatekeeper Self-Efficacy Scale at baseline, immediately after taking the program, and 2 months after the survey after completing the program follow-up. To compare the primary outcomes, a t test, where the significance level is 5% (2-sided), will be used to test the intervention effect on an intention-to-treat basis. Results: The study was at the stage of data collection at the time of submission. We recruited participants for this study during August and September 2021, and data collection will continue until December 2021. The data analysis related to the primary outcome will start in December 2021, and we hope to publish the results in 2022 or 2023. Conclusions: This is the first study to investigate the effectiveness of an online GKT program for postsecondary students to improve self-efficacy as a gatekeeper using a randomized controlled trial design. The study will explore the potential of an online peer gatekeeper program for postsecondary students that can be disseminated online to a large number of students with minimal cost.