Objective The purpose of this study was to develop an instrument for assessing triggers of suicidal ideation among older outpatients and determine its psychometrics. Method Participants were recruited from older outpatients of two hospitals in northern Taiwan. The 34‐item Triggers of Suicidal Ideation Inventory (TSII) was developed, and its items were validated by experts in two runs of Delphi technique survey. Pre‐testing this TSII in 200 older outpatients resulted in a 12‐item TSII, with three items not considered triggers but suggesting the need for further psychiatric assessment. The 9‐item TSII was examined by criterion validity, construct validity, internal consistency reliability, and test–retest reliability. Results Factor analysis of the final version of TSII yielded a three‐factor solution, accounting for 68.75% of the variance. Participants with depressive tendency tended to have higher TSII scores than participants with no depressive tendency (t = 8.62, p < 0.01), indicating good construct validity. TSII scores were significantly and positively correlated with Beck Scale for Suicide Ideation (r = 0.45, p < 0.01) and UCLA Loneliness (r = 0.55, p < 0.01) scores, indicating satisfactory criterion validity. The TSII had Cronbach's α and intraclass correlation coefficient of 0.70 and 0.99, respectively, indicating acceptable internal consistency reliability and excellent test–retest reliability. Receiver operating characteristic analysis revealed an area under the curve of 0.82, indicating excellent ability to detect triggers of suicidal ideation. With a cutoff point of 2, the sensitivity and specificity were 0.76 and 0.69, respectively. Conclusions The TSII can be completed in 5 min, is perceived as easy to complete, and yielded highly acceptable parameters of validity and reliability. Copyright © 2016 John Wiley & Sons, Ltd.