ABSTRACT Objective To examine trends in inpatient length of stay and identify factors associated with increasing inpatient length of stay across diagnostic categories, using a state‐wide linked data cohort over 8 years. Methods This retrospective cohort study utilised linked data from New South Wales (NSW) public hospitals between 2015 and 2022, encompassing over 4.5 million acute inpatient episodes. Long and prolonged inpatient length of stay was defined by 90th and 95th percentiles respectively across the study population. Demographic and clinical factors associated with increased inpatient length of stay were determined using multivariable ordinal logistic regression. Results Across 4.5 million admissions analysed between 2015 and 2022, long (9–16 days) and prolonged (> 16 days) inpatient length of stay increased by 14% and 33%, respectively. Important factors associated with increasing inpatient length of stay included higher comorbidity (adjusted odds ratio [OR] 6.61 (95% CI 6.56, 6.66)) and inpatient admission for mental health disorders (adjusted OR 5.32 (95% CI 5.26, 5.38)) and older age > 80 years (adjusted OR 2.86 (95% CI 2.82, 2.90)). Admissions for haematological and oncology and mental health disorders had the highest proportions of long length of stay (> 9 days). Conclusion A disproportionate increase in patients with prolonged inpatient length of stay, particularly in specific diagnostic categories, comorbidities and age groups was observed which may be contributing to hospital and Emergency Department overcrowding.