Importance: Mental health of soldiers is adversely affected by the death and injury of other unit members, but whether risk of suicide attempt is influenced by previous suicide attempts in a soldier's unit is unknown. Objective: To examine whether a soldier's risk of suicide attempt is influenced by previous suicide attempts in that soldier's unit. Design, Setting, and Participants: Using administrative data from the Army Study to Assess Risk and Resilience in Servicemembers (STARRS), this study identified person-month records for all active-duty, regular US Army, enlisted soldiers who attempted suicide from January 1, 2004, through December 31, 2009 (n = 9650), and an equal-probability sample of control person-months (n = 153 528). Data analysis was performed from August 8, 2016, to April 10, 2017. Main Outcomes and Measures: Logistic regression analyses examined the number of past-year suicide attempts in a soldier's unit as a predictor of subsequent suicide attempt, controlling for sociodemographic features, service-related characteristics, prior mental health diagnosis, and other unit variables, including suicide-, combat-, and unintentional injury-related unit deaths. The study also examined whether the influence of previous unit suicide attempts varied by military occupational specialty (MOS) and unit size. Results: Of the final analytic sample of 9512 enlisted soldiers who attempted suicide and 151 526 control person-months, most were male (86.4%), 29 years or younger (68.4%), younger than 21 years when entering the army (62.2%), white (59.8%), high school educated (76.6%), and currently married (54.8%). In adjusted models, soldiers were more likely to attempt suicide if 1 or more suicide attempts occurred in their unit during the past year (odds ratios [ORs], 1.4-2.3; P < .001), with odds increasing as the number of unit attempts increased. The odds of suicide attempt among soldiers in a unit with 5 or more past-year attempts was more than twice that of soldiers in a unit with no previous attempts (OR, 2.3; 95%CI, 2.1-2.6). The association of previous unit suicide attempts with subsequent risk was significant whether soldiers had a combat arms MOS or other MOS (ORs, 1.4-2.3; P < .001) and regardless of unit size, with the highest risk among those in smaller units (1-40 soldiers) (ORs, 2.1-5.9; P < .001). The population-attributable risk proportion for 1 or more unit suicide attempts in the past year indicated that, if this risk could be reduced to no unit attempts, 18.2% of attempts would not occur. Conclusions and Relevance: Risk of suicide attempt among soldiers increased as the number of past-year suicide attempts within their unit increased for combat arms and other MOSs and for units of any size but particularly for smaller units. Units with a history of suicide attempts may be important targets for preventive interventions.