Effective teamwork is integral to the functioning of many professions. Employers desire work-ready graduates who have developed the so-called 'soft skills' involved in teamwork. One way for students to practise and gain feedback on their team skills is through team-based assessment tasks. The TeamUP Rubric was developed to teach and assess five domains of teamwork skills through peer feedback. Initial evaluation of the five domains and use of the TeamUP with health and arts students has provided initial support for its utility as a measure of teamwork. In the current study the scale structure of each of the five domains was evaluated using Mokken scale analysis (MSA). MSA is a non-parametric item response theory approach that evaluates the internal structure of an evaluation tool. One-hundred and seventy-seven primary education students were recruited for the study. All participants were undertaking a teamwork assignment in a unit of study in their education degree at an Australian regional university. They completed the TeamUP Rubric assessment for themselves and for each of the students in their team. Each TeamUP domain met the requirements for a Mokken scale. The current study supports the validity of the scores derived from the TeamUP Rubric and further supports its use as a tool for teaching and assessing teamwork.