Despite the recent interest in discovering language teacher beliefs in general, L2 writing teachers' beliefs remain almost an untouched area. Considering this gap, this study aims at developing and validating a sound and comprehensive inventory that can be used for exploring L2 instructors' beliefs regarding the nature of L2 writing, teaching L2 writing, and assessing L2 writing. To do that, a total of 653 writing instructors were involved in this two-phase study. In the development phase, a 94-item inventory, Teachers' English Writing Beliefs Inventory (TEWBI), was devised. The factor structure of the TEWBI was determined through Exploratory Factor Analysis with the participation of 311 writing instructors. The results reveal that the TEWBI has a reliability value with .94, resulting in 57 items under three sub-scales. The Validation phase aimed at validating the structure of the inventory and its subscales, recruiting another 342 writing instructors. The results reveal that the model fits the data well (e.g. the RMSEA with .044 (90 % CI = .042-.046)), indicating the TEWBI produces psychometrically reliable and valid scores. In light of these findings, the sub-constructs of the TEWBI are discussed, and implications for L2 writing research are provided.