Abstract: Importance: The COVID-19 pandemic has raised considerable concerns about increased risk for suicidal behavior among US military veterans, who already had elevated rates of suicide before the pandemic. Objective: To examine longitudinal changes in suicidal behavior from before the COVID-19 pandemic to nearly 10 months into the pandemic and identify risk factors and COVID-related variables associated with new-onset suicide ideation (SI). Design, Setting, and Participants: This population-based prospective cohort study used data from the first and second wave of the National Health and Resilience in Veterans Study, conducted from November 18, 2019, to December 19, 2020. Median dates of data collection for the prepandemic and peripandemic assessments were November 21, 2019, and November 14, 2020, nearly 10 months after the start of the COVID-19 public health emergency in the US. A total of 3078 US military veterans aged 22 to 99 years were included in the study. Main Outcomes and Measures: Past-year SI and suicide attempts. Results: In this cohort study of 3078 US veterans (mean [SD] age, 63.2 [14.7] years; 91.6% men; 79.3% non-Hispanic White veterans, 10.3% non-Hispanic Black veterans, and 6.0% Hispanic veterans), 233 (7.8%) reported past-year SI, and 8 (0.3%) reported suicide attempts at the peripandemic assessment. Past-year SI decreased from 10.6% prepandemic (95% CI, 9.6%-11.8%) to 7.8% peripandemic (95% CI, 6.9%-8.8%). A total of 82 veterans (2.6%) developed new-onset SI over the follow-up period. After adjusting for sociodemographic and military characteristics, the strongest risk factors and COVID-19-related variables for new-onset SI were low social support (odds ratio [OR], 2.77; 95% CI, 1.46-5.28), suicide attempt history (OR, 6.31; 95% CI, 2.71-14.67), lifetime posttraumatic stress disorder and/or depression (OR, 2.25; 95% CI, 1.16-4.35), past-year alcohol use disorder severity (OR, 1.06; 95% CI, 1.01-1.12), COVID-19 infection (OR, 2.41; 95% CI, 1.41-5.01), and worsening of social relationships during the pandemic (OR, 1.47; 95% CI, 1.16-1.88). Conclusions and Relevance: The results of this cohort study suggest that despite grim forecasts that the COVID-19 pandemic would exacerbate suicidality among US military veterans, the rate of SI decreased at the population level nearly 10 months into the pandemic. Veterans who were infected with COVID-19 were more than twice as likely to report SI, which suggests the need for future research to examine the potential link between COVID-19 infection and suicidal behavior.