Background <p>The role of activity diversity across physical, cognitive, and social domains in preserving physical function remains unexplored.</p> Aims <p>To investigate the associations between multidomain activity diversity and physical function in community-dwelling older adults.</p> Methods <p>We analyzed data from 763 participants (mean age: 74.8 ± 6.2 years; 63.8% female) in the 2018 Tarumizu Study. Activity diversity was assessed using 12 items (physical, cognitive, and social activities; four per domain). High engagement was defined as participation in ≥ 3 within a domain. Participants were categorized by the number of highly engaged domains (0–1, 2, and 3). Low physical function was defined as low grip strength (&lt; 28&#xa0;kg for males; &lt;18&#xa0;kg for females) and/or slow gait speed (&lt; 1.0&#xa0;m/s). Logistic regression analyses examined cross-sectional (<i>n</i> = 763) and 5-year longitudinal (<i>n</i> = 175 among those without low physical function at baseline) associations.</p> Results <p>Cross-sectionally, low physical function occurred in 45.3%, 29.1%, and 21.4% in the 0–1, 2, and 3 domain groups, respectively. Participants engaged in two or three domains had significantly lower odds of low physical function than those engaged in 0–1 domains. Over 5 years, incident low physical function occurred in 35.1%, 16.2%, and 11.4% of the 0–1, 2, and 3 domain groups, respectively, with significantly lower risks in the 2 and 3 domain groups, and a linear trend (<i>p</i> for trend = 0.023).</p> Discussion <p>Engagement in ≥ 2 activity domains was associated with reduced risk of low physical function in older adults.</p> Conclusions <p>Maintaining diverse lifestyle activities supports physical function in later life.</p>