The study examined the relationship between self-determination and mathematics achievement among grade 12 learners in secondary schools in Kenya. The ex post facto research design was used. A sample size of 445 grade 12 learners was obtained. The self-determination scale was used to collect data. The findings indicate a significant influence, though weak, positive correlation (r = 0.142, n = 396, p = 0.005) between self-efficacy of expectation and mathematics achievement; a significant, though weak, positive correlation (r = 0.182, n = 396, p < 0.05) between internal locus of control and mathematics achievement; very weak positive correlation (r = 0.088), but no significant correlation [n = (396) r = 0.088, p = 0.080 (ns)] between the level of learner self-knowledge and mathematics achievement; and finally, a weak, positive correlation [n = (396), r = 0.192, p < 0.05] between learner level of self-instruction and mathematics achievement. The study recommends that teachers should play a role in helping learners develop the self-determination resources by providing autonomy-supportive classrooms.