This study aimed to evaluate the effect of Structured Water Dance Intervention (SWAN) on muscular hypertonia in individuals with profound intellectual and multiple disability (PIMD). Muscular hypertonia has a multitude of negative consequences for people with PIMD because it can lead to contractures, pain, mobility impairment, pressure ulcers that limits functional behavior as well as gross and fine motor function. Thirty-six individuals with PIMD in four Swedish regions were randomized to two groups in a multicenter, crossover design. Two withdrew participation, thus 34 individuals completed the intervention. The intervention was administered for 40 min once a week during a 12-week period. Outcomes related to muscular hypertonia were examined using the Modified Ashworth Scale (MAS), and based on accompanying assistants' assessments. Hypertonia decreased from baseline to the end of the intervention period, as demonstrated by a decrease in MAS score. Hypertonia also decreased during the sessions, as shown by the assistants' ratings. In conclusion, this study demonstrates that SWAN holds potential to reduce muscular hypertonia in people with PIMD and points out the importance of customized physical treatment alternatives. The study provides useful information for the design of future non-invasive, non-pharmacological interventions to reduce muscular hypertonia in PIMD.