Blended learning environments demand teachers possessing a high level of assessment literacy, as they play a crucial role in facilitating students' second language (L2) learning outcomes and overall well-being. However, a notable research gap exists in understanding L2 teacher assessment literacy within the blended learning context, further exacerbated by the scarcity of valid assessment instruments tailored specifically to investigate L2 teachers' assessment literacy for blended learning. To address this gap, the present study undertook the development and validation of the L2 Teachers' Blended Assessment Literacy Scale (L2TBALS). The study involved 614 L2 teachers from China, divided into two sub-samples. The first sample underwent exploratory factor analysis (EFA), which revealed a robust four-factor solution comprising Knowledge, Attitude, Practice, and Socio-emotional management. Confirmatory factor analysis (CFA) with the other sample of 307 L2 teachers confirmed the scale's structure and sub-scales, establishing the final 40-item L2TBALS as a psychometrically reliable and valid instrument for measuring selfreported L2 teachers' blended assessment literacy. These findings have practical implications for teachers and teacher education programs, as well as broader implications for assessment literacy research.