This study sought to investigate the relationship between language mindsets and grammar learning strategies and grammatical performance among English as L2 and L3 learners. Moreover, this study examined fixed and growth language mindsets across gender and language groups. The sample included 320 (N[subscript L2] = 160, N[subscript L3] = 160) intermediate Iranian EFL learners who responded to self-report scales ta as well as a grammar test. Independent samples t-tests indicated that there were significant differences between mindsets scores of male and female L2 learners. The results also revealed that L2 learners endorsed more growth mindsets than L3 learners. Finally, path analysis showed that language mindsets significantly predicted grammar learning strategies and grammar scores of both L2 and L3 learners.