Objective This study aimed to develop and verify an endometriosis self-assessment tool (ESAT). Methods A non-experimental, descriptive, correlational study design was used. Candidate items were developed based on a conceptual framework constructed using the results of in-depth interviews and an integrative literature review. The construct validity of the developed tool was also examined. One-hundred and forty-two participants (117 patients with endometriosis and 25 patients without endometriosis) were included in the validity and reliability tests. The data were collected between August and December 2018. Nomological validity was verified based on significant correlations between the ESAT and the quality-of-life scores. Results A 21-item ESAT was developed, and its construct validity was supported. Exploratory factor analysis indicated that the tool consisted of four components (gastrointestinal symptoms, dysmenorrhea, usual symptoms, and the amount and characteristics of menstrual bleeding) with a variance of 61.6%. The variance in quality-of-life scores, as explained by the ESAT scores, was relatively high. Receiver operator characteristics curve analysis indicated that ESAT scores significantly differentiated endometriosis from non-endometriosis with fair discriminatory power at a cut-off score of 50 (sensitivity, 0.76; specificity, 0.72; area under the curve, >0.75; P50 points were more likely to have endometriosis. Thus, the reliability of the ESAT was confirmed. Conclusion The devised tool appears valid and reliable. This tool may allow women to determine their risk of endometriosis by distinguishing between normal and pathological menstruation-related symptoms.