Perfectionism, a personality trait characterized by the pursuit of flawlessness and high personal standards, and motivational regulation, the strategies through which individuals manage their motivational states, have received limited attention in second language (L2) writing. Framed within social cognitive theory, this study examines how two dimensions of perfectionism—perfectionistic strivings and perfectionistic concerns—relate to writing performance (syntactic complexity, accuracy, lexical complexity, and fluency) and how motivational regulation sub-strategies (interest enhancement, self-talk, and emotional control) mediate these relationships. Data from 689 university students in China were analyzed using questionnaires and argumentative writing samples. Results indicated that perfectionistic strivings positively predicted syntactic complexity, accuracy, and lexical complexity, while perfectionistic concerns negatively predicted these dimensions; neither dimension significantly affected fluency. Crucially, motivational regulation sub-strategies partially mediated the relations between perfectionism and writing performance. These findings underscore the importance of distinguishing perfectionism dimensions and targeting motivational regulation strategies to improve L2 writing. Implications for instruction and directions for future longitudinal research are discussed. • Perfectionistic strivings and concerns affect writing via motivational regulation. • Strivings improve syntax, accuracy, and lexical complexity; concerns hinder them. • Most motivational regulation sub-strategies mediate perfectionism’s impact on CALF. • Perfectionism influences writing through motivational regulation.