Productive struggle is an effective mathematics teaching practice that provides students opportunities to grapple with challenging mathematical concepts, connect prior knowledge, discover new ideas, and develop their critical thinking skills. Research has documented the benefits of secondary mathematics students engaging in productive struggle. Additional work is needed to detail how productive struggle relates to students' mathematical achievement, specifically in the International Baccalaureate (IB) classroom. This study examined what correlation existed between 68 U.S. grade 11 and 12 IB mathematics students' level of productive struggle exhibited during tasks of varying levels of cognitive demand and their mathematical achievement based on 10 IB established markschemes and a summative assessment. A strong, positive correlation was found between the level of productive struggle and mathematical achievement across all cognitive demand levels. These results support prior work emphasizing the importance of teachers making productive struggle a consistent, pervasive practice in their classrooms to foster student mathematical achievement and understanding.