Background <p>Externalizing problems represent a major public health concern during adolescence. Reliable and culturally valid screening instruments are essential for early identification, particularly in diverse sociocultural contexts such as Peru. This study aimed to analyze the psychometric properties of the Youth Externalizing Problems Screener (YEPS) in Peruvian adolescents. Specifically, we examined its factor structure, tested measurement invariance across ethnolinguistic and sex groups, and gathered evidence of validity based on relations to other variables, including anxiety and depressive symptomatology.</p> Methods <p>Participants were 1,233 adolescents (50.2% female; <i>M</i>_age = 14.50, <i>SD</i> = 1.50) recruited from secondary schools in Metropolitan Lima (Spanish-speaking, <i>n</i> = 705) and Cusco (Quechua-speaking, <i>n</i> = 528). Confirmatory factor analysis (CFA) and item response theory (IRT) models were applied to evaluate structural validity and item-level properties. Measurement invariance across sex and ethnolinguistic groups was tested using multi-group CFA. Evidence of validity based on relations to other variables was examined via correlations with depression (PHQ-9) and anxiety (GAD-7) scores.</p> Results <p>Findings supported a bifactor structure, with a predominant general externalizing factor and specific dimensions reflecting hyperactivity–inattention and oppositional defiance. Bifactor indices indicated that the total score was psychometrically appropriate, and IRT analyses showed high discrimination parameters. Measurement invariance was upheld across sex at all levels (configural, metric, scalar, and strict), permitting meaningful comparisons of both latent means and observed scores. For ethnolinguistic groups, configural, metric, and scalar invariance were supported; however, strict invariance was not achieved, suggesting differences in residual variances. The YEPS general factor demonstrated moderate correlations with depression (<i>r</i> = .47, <i>p</i> &lt; .001) and anxiety (<i>r</i> = .45, <i>p</i> &lt; .001).</p> Conclusions <p>The YEPS demonstrates adequate psychometric properties in Peruvian adolescents, supporting its use as a reliable tool for assessing externalizing problems in research and applied settings. While the total score adequately captures the general tendency toward externalizing problems, subscale variance may also be clinically relevant. Strict invariance across sex supports its broad applicability, whereas partial invariance across ethnolinguistic groups highlights the importance of considering cultural context when interpreting raw scores. Overall, the YEPS offers a culturally sensitive and psychometrically sound screening option for adolescent populations in diverse Peruvian settings.</p>