Teaching method is a major factor that affects students' motivation to learn physics. This study investigated the effects of using mastery learning approach (MLA) on secondary school students' motivation to learn physics. Solomon four non-equivalent control group design under the quasi-experimental research method was used in which a random sample of 4 co-educational secondary schools was obtained in Kieni East Division of Nyeri District in Kenya. The 4 schools were randomly put into 4 groups. Each school provided 1 Form Two class for the study; hence, a total of 161 students were involved. The students were taught the same physics content. In the experimental groups, MLA teaching method was used while the regular teaching method was used in the control groups. The researchers trained the teachers in the experimental groups on the technique of MLA before the treatment. Two groups were pre-tested prior to the implementation of the MLA treatment. At the end of treatment period, all the 4 groups were post-tested using a validated Students' Motivation Questionnaire, whose reliability coefficient was 0.76. Data were analysed using the t test, analysis of variance and analysis of covariance. The results of the study show that students exposed to MLA have significantly higher motivation than those taught through regular methods. Gender has no significant influence on their motivation to learn physics. The researchers conclude that MLA is an effective teaching method in motivating students; hence, physics teachers should incorporate it in teaching.