This study investigated the simultaneous impact of demographic, personality, intelligence, and (prior) study performance factors on students' academic achievement in a three-year academic problem-based psychology program. Information regarding students' gender, age, nationality, pre-university education, high school grades, Big Five personality traits, intelligence (i.e., numerical, verbal, spatial), observed learning activities, and self-study time was collected among almost 1800 students enrolled in this psychology bachelor program at Erasmus University Rotterdam between 2003 and 2009. Academic achievement was measured by students' yearly number of acquired credit points, a course test, and a knowledge progress test. Multiple regression analyses showed that observed learning activities, first- and second-year performance, high school grades, conscientiousness, and verbal ability were most strongly and consistently related to academic achievement in the bachelor. Other student factors also contributed to academic achievement, but their influence was less prominent and mainly restricted to the first year. These results suggest that (prior) educational achievements and observable learning activities are most important for academic success in a problem-based learning bachelor program. (Contains 5 tables.)