Students’ development of multilingual competence has attracted increasing attention from language researchers and educators. However, research on students’ interaction strategy use in group discussions across different language settings remains scarce. In this study, therefore, we examined interaction strategy use in Cantonese as a first language (L1) and Putonghua as a second language (L2) during group discussion tasks among 42 primary school students in Hong Kong. We also investigated the effects of interaction strategy use on performance in respective tasks. We discovered that students employed significantly more interaction strategies in L1 than in L2, with a higher contribution to L1 task performance. Specifically, three of the five strategies identified—Strategy 2 (S2) asking for opinions, S3 expressing attitude, and S5 non-verbal language—were employed more frequently in L1 than in L2. Furthermore, we found that strategy use had various effects on oral performance between the two languages. In the L1 task, S1 expressing actively, S3 expressing attitude, and S4 giving clarification significantly predicted students’ group discussion performance, whereas this effect was only observed in S1 expressing actively in the L2 task. Pedagogical implications for primary students’ learning of interaction strategies for group discussions in both L1 and L2 are discussed. Copyright © 2024 John Wiley & Sons Ltd.