<p>According to Self-Determination Theory, it is essential to experience autonomy, competence, and relatedness in order to develop intrinsic motivation and motivational beliefs such as self-efficacy. Posing and solving one’s own modelling problems may support these needs by offering opportunities for choice, aligning problems with individual learning needs, and fostering peer interactions. We investigated (a) whether posing and solving one’s own problems affects self-efficacy; (b) whether posing and solving one’s own problems affects experiences of autonomy, competence, and relatedness; (c) whether these experiences affect self-efficacy, (d) whether these experiences mediate the effect of the intervention on self-efficacy, and (e) whether prior self-efficacy moderates the effect of the intervention on basic needs. In an experimental study, 194 tenth-grade students were randomly assigned to either pose and solve their own modelling problems or solve given modelling problems. Results showed no total effect of the intervention on self-efficacy. However, students who posed and solved their own problems reported higher experiences of autonomy and relatedness but not competence. These effects were primarily attributable to the problem posing activity and not the activity of solving their own problems. Experiences of autonomy, competence, and relatedness were positively associated with self-efficacy. Only autonomy mediated the effect of problem posing on self-efficacy. Prior self-efficacy did not moderate the impact of the intervention on students’ experiences of autonomy, competence, or relatedness. The findings contribute to research on motivation and problem posing by highlighting the potential of modelling-related problem posing to support basic psychological needs and indirectly foster self-efficacy in mathematics.</p>