Importance  During the past decade, cannabis use among US adults has increased markedly, with a parallel increase in suicidality (ideation, plan, attempt, and death). However, associations between cannabis use and suicidality among young adults are poorly understood.

Objective  To determine whether cannabis use and cannabis use disorder (CUD) are associated with a higher prevalence of suicidality among young adults with or without depression and to assess whether these associations vary by sex.

Design, Setting, and Participants  This survey study examined data from 281 650 adults aged 18 to 34 years who participated in the National Surveys on Drug Use and Health. Data were collected from January 1, 2008, to December 31, 2019.

Exposures  Prevalence of past-year daily or near-daily cannabis use (≥300 days in the past year), CUD, and major depressive episode (MDE). Past-year CUD and MDE were based on DSM-IV diagnostic criteria.

Main Outcomes and Measures  Past-year suicidal ideation, plan, and attempt.

Results  Among the 281 650 adults aged 18 to 34 (men, 49.9% [95% CI, 49.6%-50.2%]; women, 50.1% [95% CI, 49.8%-50.4%]) included in the analysis, past-year suicidal ideation and plan along with daily cannabis use increased among all examined sociodemographic subgroups (except daily cannabis use among current high-school students), and past-year suicide attempt increased among most subgroups. National trends in adjusted prevalence of past-year suicidal ideation, plan, and attempt varied by daily and nondaily cannabis use and CUD among adults with or without MDE. After controlling for MDE, CUD, cannabis use status, and potential confounding factors, the adjusted prevalence of suicidal ideation, plan, and attempt increased 1.4 to 1.6 times from the 2008-2009 to 2018-2019 periods (adjusted risk ratio [ARR] for suicidal ideation, 1.4 [95% CI, 1.3-1.5]; ARR for suicide plan, 1.6 [95% CI, 1.5-1.9]; ARR for suicide attempt, 1.4 [95% CI, 1.2-1.7]), with 2008 to 2009 as the reference period. Past-year CUD, daily cannabis use, and nondaily cannabis use were associated with a higher prevalence of past-year suicidal ideation, plan, and attempt in both sexes (eg, among individuals without MDE, prevalence of suicidal ideation for those with vs without CUD was 13.9% vs 3.5% among women and 9.9% vs 3.0% among men; P < .001), but significantly more so in women than men (eg, suicide plan among those with CUD and MDE was 52% higher for women [23.7%] than men [15.6%]; P < .001).

Conclusions and Relevance  From 2008 to 2019, suicidal ideation, plan, and attempt increased 40% to 60% over increases ascribed to cannabis use and MDE. Future research is needed to examine this increase in suicidality and to determine whether it is due to cannabis use or overlapping risk factors.