IntroductionBeing able to write is a key competency in educational models; therefore, it is important to have a scale to assess writing self-efficacy.ObjectiveThe study evaluated the internal structure, providing evidence of reliability, validity, and factorial invariance of the self-efficacy for writing scale (SEWS) across sex and age in Peruvian schoolchildren.MethodsAn instrumental study was carried out on 1,107 Peruvian adolescents (early, middle, and late) of school age. A confirmatory factor analysis was carried out and the factorial invariance for sex and age was evaluated.ResultsA good fit was obtained for the total sample (χ2 = 1229.08, df = 101, CFI = 0.997, TLI = 0.997, RMSEA = 0.057 [90% CI: 0.0540.059], and SRMR = 0.029) and presented good α, ω, and H reliability (>0.89). Correlations between another measure of writing self-efficacy (p = 0.76; p < .01), study satisfaction (p = 0.31; p < .01), and burnout (p = −0.12; p < .01) scales supported convergent and concurrent validity. Multigroup factor analysis supported strict invariance for sex and age, for which the SEWS provides evidence of validity and reliability.DiscussionThis adaptation of the SEWS is a valid, reliable, and invariant measure that can be considered for assessing self-efficacy for writing in Peruvian high school students.