It is generally accepted and well documented that mastery goal orientation positively affects students' learning behavior. However, less is known about this association in self-directed learning during adolescence, which additionally promotes student's positive emotions. This study tested whether positive emotions mediate the association between mastery goal orientation and effort investment, absorption, elaboration, and self-control in self-directed vs. teacher-directed learning in a sample of 824 (M[subscript age] = 13.36; SD = 0.79) 7th and 8th grade students in Germany. Multigroup structural equation modeling and latent mean comparison were used to assess potential group differences. Positive emotions mediate all relationships between mastery goal orientation and learning behavior in students from self-directed learning, whereas positive emotions only mediate the relations between mastery goal orientation and absorption in students from teacher-directed learning.