Emotions have received scant attention in L2 writing research except anxiety, leaving their role largely unclear. This study was designed mainly to examine the contributions of anxiety, enjoyment, and boredom to writing achievement. We also aimed to develop and validate instruments to measure foreign language writing enjoyment and boredom. Two sub-studies were conducted among three groups of junior secondary EFL learners in China (N 1 = 310; N 2 = 326; N 3 = 1036). In Study 1, the Foreign Language Writing Enjoyment and Boredom Scales were developed based on the responses from Group 1 to open-ended questions. Both scales were administered to Groups 2 and 3. Group 3 also filled in the questionnaires for criterion variables (i.e., second language writing anxiety, foreign language anxiety, enjoyment, boredom, and burnout). Reliability and validity tests confirmed the sound psychometric properties of the two scales (e.g., internal consistency, construct/convergent/discriminant/criterion validity, and measurement invariance). In Study 2, we further obtained data on the English writing achievement of Group 3 operationalized as scores in two writing proficiency tests and an end-term exam, and self-ratings for writing proficiency. Regression analyses show that boredom had the strongest negative effects on writing achievement, followed by the positive effects of enjoyment and the insignificant effects of anxiety.