An innovative mathematics game shown to be effective for low-achieving mainstream students is tested in special education for learners with moderate or severe intellectual disabilities in Sweden, to investigate if the game also can be effective for this group of students. The game relies on a graphical, intuitive representation for numbers and arithmetic operations to foster conceptual understanding and numbers sense, and provides a set of 2-player games to develop strategic thinking and reasoning skills.  The game runs on computers and interactive white boards, and as an augmented reality application at a science centre. The study enrolled 3 teachers and 8 students in 5th to 8th grade with intellectual disabilities who played the game between 4 months and 2 years, one student with Asperger syndrome, and over 300 students in mainstream education as comparison. We compare the use of the game in special education and mainstream education with respect to usage, performance levels and learning gains. Collected data include game playing logs for all students where playing behaviour, performance and progression data was analysed; class room and science centre observations where interaction, collaboration and communication was analysed, and on in-depth interviews with the teachers. Conclusion is that the game in combination with dedicated teachers can be very effective for students with intellectual disabilities, and can result in substantial gains in mathematical understanding and strategic thinking as well as in communication skills, given time and proper support.