While COVID-19 impacted all aspects of health care and patient treatment, particularly for patients with mental health/substance use (MH/SU) concerns, research has suggested a concerning increase in the use of restraint and seclusion (R/S) interventions, although results vary depending on facility type and patient population. Thus, the present study sought to explore COVID-related changes in the use of R/S interventions among patients presenting to Ontario emergency departments (EDs) with MH/SU complaints. To determine whether temporal and clinical factors were associated with changes in R/S use during COVID, binary logistic regression models were computed using data from the National Ambulatory Care Reporting System database. We then compared both prevalence rates and probability of an R/S event occurring during an ED visit in Ontario before and after the onset of COVID. The number of ED visits during which an R/S event occurred for patients presenting with MH/SU concerns increased by 9.5%, while their odds of an R/S event occurring during an ED visit increased by 23% in Ontario after COVID onset. Similarly, R/S event probability increased for patients presenting with MH/SU concerns after COVID onset (0.7% - 21.3% increase), particularly during the first wave, with the greatest increases observed for concerns associated with increased restraint risk pre-COVID. R/S intervention use increased substantially for patients presenting to Ontario EDs with MH/SU concerns during the first wave of COVID when the strain on healthcare system and uncertainty about the virus was arguably greatest. Patients with concerns already associated with increased R/S risk also showed the largest increases in R/S probability, suggesting increased behavioural issues during treatment among this population after COVID onset. These results have the potential to inform existing policies to mitigate risks associated with R/S intervention use during future public health emergencies and in general practice.