<p>Transgender and gender diverse (TGD) youth are at increased risk of co-using alcohol and cannabis and experiencing related harms due to gender minority stress (GMS). This study aimed to characterize alcohol and cannabis co-use and related harms among TGD youth. Forty Canadian TGD youth who drink completed a 21-day ecological momentary assessment (EMA) period where they reported their substance use, corresponding harms, and GMS and resilience experienced each day. Participants reported 180 alcohol-only days, 104 cannabis-only days, and 41 co-use days during the EMA period. The number of alcoholic drinks consumed did not differ between co-use days and alcohol-only days, but the number of substance-related harms was greater on co-use days relative to alcohol-only days. On days when youth reported relatively greater GMS, they were more likely to engage in co-use than alcohol-only use. On days when youth reported relatively greater resilience (i.e., pride, community connectedness), they were more likely to co-use than cannabis-only use. Co-use should be considered when developing substance use policy and treatments for TGD youth.</p>