This study aimed to determine the effect of a computer-assisted instruction method using GeoGebra on achievement of prospective secondary mathematics teachers in the definite integral topic and to determine their opinions about this method. The study group consisted of 35 prospective secondary mathematics teachers studying in the mathematics education program at a state university in Turkey. The study was carried out using an embedded design, and the Definite Integral Knowledge Test and an opinion form were used for data collection. Upon analyzing the data, the computer-assisted instruction method using GeoGebra was found to positively contribute to the success of teaching the definite integral topic. Prospective teachers stated that this method should be used in math courses as it creates a fun and interesting environment with dynamic learning elements, provides visualization and opportunities to learn mathematics through practice and exercises, enables thorough understanding and explication of skills, and makes way for conceptual learning instead of memorizing. Furthermore, this study was found to facilitate conceptual learning of the relationship between the lower sum, upper sum and Riemann sum.