OBJECTIVES: This study aimed to compare mortality rates and length of hospital stay between patients with critical COVID-19 transferred to another hospital due to capacity constraints and those who remained at their initial admission hospital. DESIGN: Single-centre cohort study. SETTING AND PARTICIPANTS: 665 patients were treated for SARS-CoV-2 at two intensive care units (ICUs) in Stockholm, Sweden, from 1 March 2020 to 30 June 2021. Data on interhospital transfers (IHTs) were retrieved from medical records and patient data management systems according to predefined protocols. MAIN OUTCOME MEASURES: The outcomes were 30-day and 90-day mortality, days alive and out of ICU. HR with 95% CI were calculated using Cox proportional hazard models with adjustments for age, sex, body mass index, severity of illness, comorbidity, invasive ventilation, treatment limitations and pandemic waves. RESULTS: Of 665 patients, 133 (20%) were transferred to another hospital. The mortality rate of transferred patients compared with non-transferred patients at 30 days was 19% vs 26% (p=0.13) and at 90 days 26% vs 30% (p=0.43). In the adjusted Cox regression analysis, IHT was associated with a lower mortality risk at 30 days (HR 0.47, 95% CI 0.30 to 0.76) and 90 days (HR 0.52, 95% CI 0.34 to 0.79). However, the number of days alive and out of ICU was significantly lower for the IHT group at 30 days. CONCLUSION: In our study, IHT due to capacity constraints among critically ill COVID-19 patients was not associated with a higher mortality risk. The suitability for transfer was likely associated with lower mortality, although residual confounding cannot be ruled out. The requirement for invasive ventilation among transferred patients might account for the extended length of ICU stay, rather than the transfer itself. However, the difficulty in studying this issue lies in the fact that while patients are likely exposed to risks during transfer, they are simultaneously the patients stable enough to be transported.