Many large-scale, school-based interventions have attempted to improve academic performance through promoting students' growth mindset, defined as the belief that one's intellectual ability can increase with practice and time. However, most have shown weak to no effects. Thus, it is important to examine how growth mindset might affect retention and transfer of learning, as well as process-related variables such as cognitive load. In a double-blind, randomized controlled experiment based on 138 secondary school students, the effects of an experimentally induced growth mindset belief were examined during a learning phase in a classroom setting. Participants in the growth mindset condition perceived a lower intrinsic load and extraneous load and performed better on retention and transfer tests. Students with some prior knowledge also reported a higher mastery goal orientation. Supplementary mediation analysis suggested that the effect on transfer could be fully accounted for by changes in cognitive load perceptions. Future interventions may benefit from designs that promote motivational beliefs that reduce intrinsic and extraneous cognitive load perceptions.