Objectives To determine whether a dementia wellbeing service (DWS) signposting people with dementia to community services decreases the rate of avoidable hospital admissions, in-hospital mortality, complexity of admissions (number of comorbidities) or length of stay. Methods Interrupted time series analysis to estimate the effects of the DWS on hospital outcomes. We included all unplanned admissions for ambulatory care sensitive conditions (‘avoidable hospital admissions’) with a dementia diagnosis recorded in the Hospital Episode Statistics. The intervention region was compared with a demographically similar control region in the 2 years before and 3 years after the implementation of the new service (October 2013 to September 2018). Results There was no strong evidence that admission rates reduced and only weak evidence that the trend in average length of stay reduced slowly over time. In-hospital mortality decreased immediately after the introduction of the dementia wellbeing service compared to comparator areas (x0.64, 95% CI 0.42, 0.97, p = 0.037) but attenuated over the following years. The rate of increase in comorbidities also appeared to slow after the service began; they were similar to comparator areas by September 2018. Conclusions We found no major impact of the DWS on avoidable hospital admissions, although there was weak evidence for slightly shorter length of stay and reduced complexity of hospital admissions. These findings may or may not reflect a true benefit of the service and require further investigation. The DWS was established to improve quality of dementia care; reducing hospital admissions was never its sole purpose. More targeted interventions may be required to reduce hospital admissions for people with dementia.