Purpose: Weight Bias Internalization (WBI) is pervasive and potentially damaging for health. Little is known about WBI in youth. As negative effects of WBI have been observed when controlling for BMI, measures that allow WBI to be assessed across different weight categories are needed. The Modified Weight Bias Internalization Scale (WBIS-M) is one of the most frequently used scales in this field. Our purpose was to obtain a Spanish validated version of the WBIS-M for adolescents across different weight statuses. Methods: The data were collected from 298 secondary students (mean age 14.31; 48.32% girls; 18.8% were overweight and 6.4% had obesity). Internal structure was examined by a cross-validation analysis, using both exploratory and confirmatory factor analyses in different subsamples. Results: Item 1 showed a psychometric anomalous functioning and was deleted. The one-factor structure of the 10-item version was confirmed with adequate fit ([EFA (KMO = 0.915, χ2(55) = 1075.633, p < 0.0001)]; [CFA (χ2(35) = 200.515; GFI = 0.995; PGFI = 0.992; NFI = 0.991; SRMR = 0.060)]). Internal consistency was high (α= 0.93 ; ω = 0.93). Significant correlations with the same set of external variables assessed in the original version (anti-fat bias, self-esteem, mood, body dissatisfaction, drive for thinness, binge eating), all of them correlates of WBI in adolescents, were found. Girls and participants with obesity obtained higher scores. Conclusion: The results provide support for the validity and reliability of our WBIS-M version for use with adolescents across weight categories in Spain. Level of evidence: Level III, evidence obtained from well-designed cohort studies.