Background. Multimorbidity is recognised internationally as having a serious impact on health outcomes. It is associated with reduced quality of life, increased healthcare utilisation and future functional decline. Physical activity is associated with good health and psychological well-being. The aim of this study was to identify the impact of physical activity and physical function on the development and worsening of multimorbidity over time. Methods. Using The Irish Longitudinal Study on Ageing (TILDA), we analysed 4,823 participants >50 years with and without multimorbidity. Multimorbidity was defined as the presence of ≥2 chronic conditions. Development of multimorbidity was measured as the accrual of additional conditions over a two year period. Physical activity and physical function were measured using the International Physical Activity Questionnaire (IPAQ), gait speed (m/sec) and grip strength (kg). Results. Sixteen groups of chronic conditions were included in analyses. 53.7% of included participants had multimorbidity at baseline and 71.7% at follow up. 638/2092 (30.4%) participants without multimorbidity and 1005/2415 (41.6%) with existing multimorbidity developed new condition/s. Gait speed (RR=0.67, CI:0.49-0.90), grip strength (RR=0.98, CI:0.97-0.99) and age (compared to 50-59 years, 60-69: RR=1.30, CI:1.11-1.52; ≥70: RR=1.35, CI:1.03 – 1.77) were significantly associated with development of multimorbidity and accrual of additional conditions. Conclusion. These results show that physical function is associated with the development and worsening of multimorbidity over time. They support the recent National Institute for Health & Care Excellence (NICE) Guidance on multimorbidity that suggests that patients with multimorbidity and reduced gait speed should be identified and targeted for interventions to improve health outcomes.