As countries around the world seek to increase student achievement and engagement in mathematics, inquiry-based learning (IBL) has become increasingly popular (Artigue & Blomhøj, 2013). The purpose of this research was to explore how a group of year five students perceived engagement and problem-posing during an IBL mathematical problemposing investigation, and to explore the instructional strategies that supported the students in problem-posing. This qualitative, single instrumental case-study involved 17 students (9- to 10-year-olds) and one teacher from an independent school in South-East Queensland, Australia. The researcher led a two-week, open, mathematical investigation, which required students to develop their own investigation questions based on a video prompt. Data were collected through video observations, student work samples, and semi-structured interviews with the students. Thematic analysis and triangulation of the data revealed three themes related to engagement: collaborative learning, enjoyment and interest, and cognitive engagement and transfer of learning, and one theme related to problem-posing; confidence. The findings suggest that the IBL mathematical problem-posing investigation was behaviourally, emotionally, and cognitively engaging for the students because it created a learning environment for the students to collaboratively work with their peers, make autonomous choices about their learning, build peer relationships, and challenge themselves. Additionally, the novelty of the experience played a role in student engagement during the IBL investigation. The findings also suggest that although the students felt confident to problem-pose, they required teacher support and scaffolding to make connections between the video and mathematics, and to mathematise their ideas. Implementing the IBL mathematical problem-posing investigation was beneficial for student engagement in mathematics; however, further research is required to examine the intricates of this type of investigation, and its impact on learning.