Despite numerous studies involving personality traits and second language (L2) learning achievement over many years, there is a lack of an overall picture about how personality traits are related to L2 learning achievement. This study aims to conduct a systematic quantitative synthesis of the studies that examined the relationships between the Big Five personality traits and L2 learning achievement. A total of 137 correlation coefficients from 31 primary studies conducted in 24 countries, with a total cumulative sample size of 8853 and published between 1982 and 2020, were included in this synthesis. The findings showed that openness to experience (r = .23; 95% CI: .15, .30; p < .001), conscientiousness (r = .18; 95% CI: .08, .28; p = .002), extraversion (r = .12; 95% CI: .02, .21; p = .017), and agreeableness (r = .10; 95% CI: .01, .18; p = .025) had positive correlations with L2 learning achievement, while neuroticism (r = − .04; 95% CI: − .09, .02; p = .227) had a negative yet statistically non-significant correlation with L2 learning achievement. More specifically, openness to experience and conscientiousness were the stronger correlates with L2 learning achievement, followed by more moderate correlates of extraversion and agreeableness, while neuroticism was the weakest among the five. Furthermore, several study features (i.e., study region, age of participants, L1 and L2 similarities, and schooling levels) were shown to explain the variations in the relationships between the Big Five personality traits and L2 learning achievement across individual studies. Implications for L2 teaching and future research are discussed.