While interest in multilingualism is growing, in the field of motivation little research has examined the role played by learners' multilingual selves in constructing their motivation to learn a foreign language other than English (LOTE). Based on a dynamic model of the L2 Motivational Self System, this study explores how Chinese undergraduates' LOTE motivation is constructed temporally and contextually. In particular, it examines how motivation for being multilingual influences the formation of LOTE motivational trajectories. A longitudinal qualitative research design, which focused on learners' introspection and behaviours, was adopted to reveal the complexity of LOTE motivation. Interviews, written journals and class observation were employed to collect data at one Chinese university over a course of one academic year. Findings suggest that learners' motivation was constructed through the interplay between their future language selves and current learning experiences, during which they interacted with their situated contexts and negotiated the value of learning a LOTE. Notably, during this process the development of a multilingual motivation was revealed to be crucial for learners to understand the significance of LOTE learning and strengthen their LOTE motivation. Implications emphasise the importance of language educators helping learners to obtain cross-linguistic experiences and construct ideal multilingual selves.