Background: While it's clear that autonomous motivation significantly boosts academic success, there are conflicting findings regarding the opposite relation. Besides, the reciprocal relations among controlled motivation and achievement present mixed results. Adequately distinguishing between variations among individuals and within individuals results key to acknowledge such relations. Aim: This longitudinal study examines the reciprocal relations between controlled and autonomous forms of motivation and academic achievement using the RI-CLPM methodology. Sample: Participants were 1042 high school students (M = 16 years, 52% male adolescents) from 16 different high schools in urban and rural areas. Methods: A random intercept cross-lagged panel model (RI-CLPM) was tested to estimate whether students' autonomous and controlled motivation predicted achievement and/or vice versa. Independent models were estimated for the two types of motivation. Results: Overall, the RI-CLPM results indicated a unidirectional relationship between autonomous motivation and achievement. As for controlled motivation, the results of RI-CLPM models showed no reciprocal relationship between this type of motivation and achievement. Conclusions: These results underline the importance of taking within- and between-person processes into account when analysing reciprocal relations and provide crucial insights for enhancing student motivation and achievement in diverse educational contexts.