Introduction: There is increasing evidence on irritability as a distal predictor of suicidal behavior. Little is known so far on irritability as a more proximal indicator, occurring immediately before suicidal behaviors. Ecological momentary assessment (EMA) has the potential to examine temporal changes of affective states at a micro-level, in a “natural” non-laboratory setting and unbiased by retrospective recall. Thereby, it might in the future improve short-term prediction of suicidal behavior. The present study seeks to investigate whether momentary irritability is associated with suicidal thoughts in adolescent in-patients. Method: Intensive EMA on irritability (I feel irritable; 0 - not at all to 100 - very much) and suicidal thoughts (Since the last prompt, did you have suicidal thoughts? yes/no; How intense were these thoughts? 0 - not intense to 100 - very intense) were collected from n = 49 adolescent in-patients. Patients were prompted hourly, twelve times a day for five consecutive weekdays. Logistic multilevel regression was used to test for associations between momentary irritability and suicidal thoughts and mixed linear regression was used to examine associations between momentary irritability and the intensity of suicidal thoughts. Lagged models were further used to test whether irritability from one EMA sampling was able to predict the presence and the intensity of suicidal thoughts in the next EMA sampling. Results: In total, n = 2066 EMA samplings were obtained, with an average of 2.78 (SD = 1.40) days of participation and 29 (SD = 18.10) EMA prompts answered by each participant. N = 33 patients (67%) reported suicidal thoughts at least once during the assessment window. Higher irritability was associated with higher odds for suicidal thoughts (OR = 1.03 [95% CI = 1.03, 1.04], p <.001) and higher intensity of suicidal thoughts (b = 0.17 [0.14, 0.21], p <.001). Further, higher irritability predicted the presence of suicidal thoughts at the next sampling, i.e. one hour later (OR = 1.02 [1.01, 1.02], p <.001), as well as two, three and four hours, but not five hours later (OR = 1.0 [0.99, 1.01], p = .48). The intensity of suicidal thoughts was predicted by irritability reported during the previous hour (b = 0.06 [0.02, 0.09], p = .001) and two hours, but not three hours before (b = 0.02 [-0.02, 0.06], p = .39). Conclusion: Self-reported states of momentary irritability were associated with the presence and intensity of suicidal thoughts. Irritability predicted the presence of suicidal thoughts in the next hour and up to four hours later as well as the intensity of suicidal thoughts up to two hours later. Across models, effect sizes were relatively small. Momentary irritability may have some potential as a proximal indicator of suicidal thoughts in adolescent in-patients. Future research will refine EMA measurement approaches of irritability and whether it ultimately serves to predict more severe suicidal behavior, including suicide attempts.