Background: Adjustment disorder (AjD) is one of the most frequently used diagnoses in psychiatry but a diagnostic definition for AjD was only introduced in release of the ICD-11. This study sought to develop and validate a new measure operationalizing the ICD-11's narrative description of AjD, and to determine the current rate of people meeting the symptoms indicative of AjD in the general population of the Republic of Ireland. Methods: The International Adjustment Disorder Questionnaire (IADQ) was constructed to measure the core diagnostic criteria of ICD-11 AjD: stressor exposure, preoccupations with, and failure to adapt to, the stressor, timing of symptom onset, and functional impairment. A nationally representative sample (N = 1,020) of adults from Ireland completed the IADQ. Results: Confirmatory factor analysis supported construct validity and the reliability estimates were excellent. The IADQ correlated strongly with depression, anxiety, and posttraumatic stress. The criteria were met by 7.0% of the sample, adjusted for other exclusionary disorders. Discussion: The IADQ is a measure based on the ICD-11's description and produces reliable scores, however it should not be used for clinical assessment until validated with clinical interviews.