Given the significance of boredom and its detrimental effects on the English as a foreign language (EFL) students' academic achievements, this study utilised an exploratory sequential mixed-methods design to initially explore the boredom factors in EFL classes among a sample of 139 university students and then develop and validate the Precursors of Students' Boredom in EFL Classes (PSBEC) in the context of Iran. Confirmatory factor analysis, based on the data from 991 university students, resulted in the confirmation of eleven-factor model of the PSBEC scales introducing eleven factors as the main precursors: (a) teaching practices, (b) excessive class control, (c) inattentive behaviour, (d) overchallenge, (e) underchallenge, (f) intrinsic values, (g) extrinsic values, (h) negative affective factors, (i) boredom proneness, (j) classroom-related factors, and (k) curriculum design. Moreover, the PSBEC scales showed good convergent validity with the Foreign Language Enjoyment (FLE) and the Perceived Teachers' Enthusiasm (PTE) scales. The PSBEC scales also negatively correlated with students' academic achievements except for boredom due to underchallenge, supporting the ecological validity. The findings help teachers assess the boredom-inducing factors in EFL classes and find ways to avoid boredom and its detrimental effects on their EFL students.