<p>Irritability is central to developmental psychopathology, especially in adolescence and young adulthood. However, existing measures do not consistently integrate tonic/phasic manifestations with trait-like and recent state-like assessment frames. This study presents the development and initial psychometric evidence of the Adolescent Trait-State Irritability Reactivity and Regulation Inventory (IRRIT-IJ). Methods: The study included 286 Brazilian participants aged 11 to 24 years. Items were developed through a narrative literature review, expert evaluation, and semantic assessment with the target population. Internal structure was examined using polychoric exploratory procedures, the Correlation Gaussian Mixture Model, and confirmatory model evaluation. Validity evidence was examined using Spearman correlations with external measures and known-groups comparisons based on self-reported gender. Results: Expert evaluation showed high agreement (Gwet’s AC1 = 0.87), and semantic evaluation supported item comprehension. The final reduced version included 26 items: 13 State items and 13 Trait items. Analyses supported a tonic/phasic two-dimensional structure for both subscales, with adequate fit and high internal consistency for State (α = 0.892; ω = 0.891) and Trait (α = 0.891; ω = 0.890). IRRIT-IJ scores were positively associated with measures of irritability, psychological distress, minor mental disorder symptoms, and emotional regulation, with stronger associations for irritability and psychopathological symptoms. Female participants showed higher observed State and Trait scores than male participants. Conclusions: Findings provide initial support for the IRRIT-IJ as a complementary self-report measure for assessing recent state-like and trait-like irritability through tonic and phasic manifestations in adolescents and young adults. Further studies should examine temporal stability, measurement invariance, discriminant validity, and independent cross-validation.</p>