Japanese learners of English as a foreign language often do not attain levels of English proficiency that allow them to conduct even simple conversations in English. If a predictor of foreign language learning outcomes were available, educators could potentially identify and support students at risk of poor outcomes. This study investigated the non-cognitive trait of grit as a possible predictor of foreign language learning outcomes in Japanese university students. An online anonymous survey was conducted at two universities in eastern Japan. In addition to demographic information such as sex, age, and year in school, respondents were asked to self-report their most recent score on a standardized test of English, such as TOEIC or TOEFL, as well as their current GPA. Additionally, participants were administered a Japanese translation of the Grit-S measure. After confirming the validity of both the data and the measure, regression analysis was conducted to determine the relationship between grit and the English proficiency test scores both before and after controlling for prior academic achievement as measured by GPA. It was found that higher grit was predictive of higher English proficiency test scores, even after controlling for GPA. After presenting these findings, the implications of these results and ideas for future research are discussed.