This study is the second part of a project on motivation for short in-class extensive reading (ER) and examined factors affecting the motivations using self-determination theory (SDT). Japanese university learners of English as a foreign language (N = 133) responded to a questionnaire designed to measure four hypothesized predictors of ER motivation (i.e., positive peer influences, perceived autonomy, competence, and relatedness) in the middle of an academic year, and five subtypes of SDT motivation for short in-class ER (i.e., intrinsic motivation, identified regulation, introjected regulation, external regulation, and amotivation) at the end of the academic year. The results of a recursive path analysis showed that: 1) perceived autonomy had a positive impact on motivation, leading to higher intrinsic motivation and identified regulation, and lower amotivation, introjected regulation, and external regulation; 2) positive peer influences had a positive impact on more self-determined types of motivation (i.e., intrinsic motivation and identified regulation); 3) perceived competence was neither associated with amotivation nor more self-determined types of motivation; and 4) perceived relatedness held no relevance for any of the five subtypes of motivation. These findings indicate the importance of perceiving autonomy and peer engagement in ER in enhancing motivation for short in-class ER.