This study aimed to provide a novel analysis of the psychometric properties of the Acceptance and Action Questionnaire-II (AAQ-II) within a diverse Colombian population, also extending its validation across more diverse populations and enhancing the generalizability of the findings. The AAQ-II is widely used to measure Psychological Inflexibility (PI), which is composed of seven items on a Likert scale. The participants were 4379 Colombian males and females. The analysis of the internal structure, quality of items, and measurement invariance (MI) of the AAQ-II was performed using a Graded Response Model (GRM). Results confirmed the unidimensional structure, which explained 71% of the variance, and showed strong factor loadings ranging from 0.80 to 0.88 across the seven AAQ-II items. The item analysis showed good values of discrimination and difficulties across the seven options of the answer of the Likert scale, suggesting that the AAQ-II effectively measures different levels of PI. The test information function (TIF) indicated that the AAQ-II provides a reliable measure for individuals with average and high PI scores. The MI results showed that there is invariance between males and females. The AAQ-II demonstrated adequate psychometric properties, including a well-defined unidimensional structure, high item discrimination, satisfactory measurement precision for average-to-above-average PI levels, and measurement invariance across gender, providing solid evidence of validity to support the interpretation of its scores.