Background: Writing self-efficacy is a central construct in educational research, grounded in Bandura’s social cognitive theory. However, most available instruments have been developed in Western and urban contexts, which limits their applicability to indigenous bilingual populations, such as Quechua-speaking students in Peru. The absence of validated scales in these contexts hinders the accurate assessment of writing self-efficacy and the implementation of educational strategies tailored to their linguistic and cultural needs. Objective: This study aimed to adapt and validate the Writing Self-Efficacy Scale (QEWSE) for Quechua-speaking students in basic education in Peru, ensuring its structural validity and reliability. Methods: An instrumental study was conducted with a sample of 265 secondary school students (50.6% female, 49.4% male), using convenience sampling. Confirmatory factor analysis (CFA) was applied to evaluate the structure of the instrument. Reliability was assessed using Cronbach’s alpha and McDonald’s omega coefficients. Results: The four first-order factor model (Ideation, Skills, Usage, and Self-Regulation) showed adequate fit indices (CFI = 0.92; TLI = 0.91; RMSEA = 0.05 [90% CI: 0.05–0.06]; SRMR = 0.05). However, the high inter-factor correlations (≥0.85) suggest the relevance of a second-order model, which demonstrated a reasonable fit (CFI = 0.92; TLI = 0.91; RMSEA = 0.06; SRMR = 0.05), supporting the interpretation of writing self-efficacy as a global construct. The scale showed high reliability across all factors (α and ω ranged from 0.74 to 0.90). Conclusions: The QEWS-S demonstrates strong psychometric properties for assessing writing self-efficacy among Quechua-speaking students. The hierarchical second-order model offers a more accurate theoretical and empirical representation, allowing for the reporting of an overall self-efficacy score while also providing specific scores for each dimension. These results support its use in bilingual and culturally diverse contexts and lay the groundwork for future research aimed at further examining discriminant validity and developing pedagogical interventions focused on strengthening students’ confidence and writing skills.