Achievement goals (AGs) are pivotal in L2 writing, but they are yet to be systematically conceptualized at the task-specific level, and their impact on writing task performance remains underexplored. To address this gap, this study conceptualized task-specific AGs, validated a measurement instrument, and examined their effects on task performance. A total of 629 Chinese undergraduate EFL learners completed argumentative writing tasks and a composite questionnaire. Exploratory structural equation modeling and confirmatory factor analysis supported a four-factor structure of the Scale for Task-Specific Achievement Goals in L2 Writing: mastery-approach, mastery-avoidance, performance-approach, and performance-avoidance. The scale demonstrated satisfactory psychometric properties (reliability, convergent, discriminant, criterion validity, and cross-gender measurement invariance). Structural equation modeling revealed that mastery-avoidance goals significantly predicted self-rated task response and performance-approach goals significantly predicted self-rated lexical resource, two dimensions of writing task performance; however, no AGs significantly predicted actual writing task performance. We argue that a certain threshold of L2 proficiency may be a prerequisite for AGs to translate into actual writing task performance. Teachers are advised to encourage adaptive AGs while simultaneously building L2 proficiency to bridge the gap between AGs and writing task performance.