Background: To date there is no standardized scale for the evaluation of self-efficacy of gatekeeper skills. Aim: The aim of this study was to develop the Gatekeeper Self-Efficacy Scale (GKSES), which consists of nine items assessing laypersons’ confidence in their own gatekeeper skills, and to confirm its reliability and validity. Method: Altogether 875 participants joined in gatekeeper training between 2010 and 2017 and completed the GKSES both before and after training. After exploratory factor analysis, we analyzed Pearson’s correlation between total GKSES scores and Attitude Toward Suicide Scale subscale scores and performed one-way analysis of variance for the score differences by demographics. Results: The GKSES is a univariate scale with good fit. Total GKSES score correlated with the appropriate attitudes for suicide prevention. Self-efficacy ratings after training improved irrespective of attribute. Participants who had had contact with suicidal individuals and those who had received previous suicide prevention training had high self-efficacy ratings, while a greater improvement was seen in the ratings of participants who had not had any previous contact or training. Limitations: It is necessary to assess the GKSES using more participants across other populations. Conclusion: The GKSES is a simple and useful measure with good internal consistency and validity for self-efficacy assessment of gatekeeper skills.