Asking questions is vital for learning, yet students seldom exhibit this behavior. Given the increasing presence of online classrooms in tertiary education, it is much needed to examine ways to encourage students to ask questions and increase their engagement. Despite the critical role of asking questions to enhance learning, little is known in research on promoting this behavior. Therefore, in this paper, we empirically tested the effect of nudging – a novel approach in education consisting of subtle interventions to change behavior – in online classrooms in tertiary education on question asking behavior, student engagement, and grades. In Experiment 1 (n = 1011), the teacher's virtual background prompted questions (prompt nudge), while in Experiment 2 (n = 449), the teacher set a goal for the students to ask one question per session (goal-setting nudge). We found a trend towards a positive effect of the prompt nudge on questions, but not on grades. Exploratory analyses revealed this was driven by students who already asked many questions. We found no effect of the goal-setting nudge on any measure, nor any effect in either experiment on student engagement. The findings demonstrate that the prompt nudge can be a possible useful and easy to implement tool to encourage questions in the online classroom.