This mixed methods study compares L2 learning motivation of Burundi simultaneous learners of French, English and Kiswahili, mainly focusing on L2 Motivational Self System (L2MSS). A total of 348 junior high school students participated in a 60-item questionnaire survey, and 12 of those learners participated in a follow-up semi-structured interview. The quantitative data were analysed using structural equation modelling (SEM) while the interview data were analysed employing a thematic analysis method. The results support the validity of the L2MSS in comparing and explaining Burundi simultaneous multiple language learners' motivation. For all the three target languages, L2 Learning Experience appeared as the strongest predictor of the learners' intended learning effort, followed by the Ideal L2 Self only in relation to English and French learning. The role of the variable of International Posture on the three components of the L2MSS, which was assumed to be influenced by the Burundi socio-educational context, was also observed in the SEM analysis. The analysis indicated that the impact of International Posture on Ideal L2 Self was only significant in the models of both English and Kiswahili while its impact on L2 Learning Experience was only significant in the model of Kiswahili. The interview data also provided further insight into the quantitative findings. Pedagogical implications are provided based on these key findings.