The validity and specifically the convergent validity of a course’s online assessments (i.e. the extent to which the online assessments really measure what is measured by other assessments meant to measure the constructs of the course’s learning outcomes) so far receive little attention, especially for business mathematics courses. Based on a business mathematics course at the authors’ university, this article aims to verify the convergent validity of an invigilated, summative online assessment by evaluating the extent to which the online assessment measures what is measured by an invigilated, summative offline assessment meant to measure the construct of the course’s learning outcomes. In addition, this article attempts to deduce whether the students performed better in the online assessment than in the offline assessment or vice versa. Findings was that such convergent validity was just scarcely acceptable and that students in the online assessment drastically outperformed themselves in the offline assessment. Reasons are proposed for the findings, for example, students’ computer anxiety, perceptions of and attitudes towards online assessments as well as some distinctive features of online assessments in mathematical courses.