In this meta-analysis, we examined the average correlations (weighted for sample size and corrected for measurement error) between the varied types of motivation specified within self-determination theory (SDT) and second language (L2) achievement. We also conducted moderator analyses to explore whether these relationships are influenced by age, educational level, proficiency level, aptitude, the language being learned, or participants' first language. The findings revealed a positive correlation between intrinsic regulation (r = 0.26, p < 0.001) and identified regulation (r = 0.16, p < 0.001) and L2 achievement. These two types constitute the global factor of autonomous motivation, which was also positively associated with achievement (r = 0.23, p < 0.001). In contrast, neither introjected regulation (r = -0.06, p = 0.06) nor external regulation (r = -0.01, p = 0.70) were related to L2 achievement, and their global factor--controlled motivation--was likewise unrelated (r = -0.03, p = 0.24). However, specific scales used to measure controlled motivation showed some variability in their associations with outcomes. Parallel analyses of motivation and language anxiety showed that intrinsic (r = -0.25, p < 0.001) and identified regulation (r = -0.18, p < 0.001) were negatively associated with anxiety, while introjected (r = 0.23, p < 0.001) and external regulation (r = 0.12, p = 0.016) were positively related. At the global level, autonomous motivation was negatively associated with anxiety (r = -0.23, p < 0.001), and controlled motivation was positively associated (r = 0.16, p = 0.001). Overall, more autonomous forms of motivation were linked to both higher achievement and lower anxiety, supporting a key proposition of SDT in L2 learning. These associations were further moderated by learners' first language, educational level, and learning context. Together, the findings put emphasis on the pivotal role of autonomous motivation in fostering better language learning outcomes.