Writing a final thesis is an important step for many students. Many thesis programs in research-oriented fields are structured as projects in which there is a one-to-one working relationship with a supervisor. In addition, the thesis' final grade is often perceived as particularly important. This research uses a cross-sectional survey design (N = 217) to assess student-perceived supervisor motivation and experienced grade pressure as predictors of student motivation when writing their thesis. Derived from expectancy-value models, valence, expectancy, and instrumentality towards achieving a good thesis grade as well as stress from the thesis work are assessed as mediating variables. The results indicate that, as hypothesized, high student-perceived supervisor motivation is associated with higher motivation components and less stress. Grade pressure is associated with higher valence, but also more stress. In turn, motivational components are positively and stress negatively associated with student motivation. The mediation analysis suggests a partially mediated positive effect of student-perceived supervisor motivation and a suppressed net zero effect of grade pressure on student motivation. Although this research cannot establish causality, the results suggest that motivating supervisors may improve student motivation in thesis writing significantly. Furthermore, the negative effects of high grade pressure may not extend to student motivation.