Background: There are comparatively few international studies investigating suicide in military veterans and no recent UK–wide studies. This is important because the wider context of being a UK Armed Forces (UKAF) veteran has changed in recent years following a period of intensive operations. We aimed to investigate the rate, timing, and risk factors for suicide in personnel who left the UKAF over a 23–year period. Methods and findings: We carried out a retrospective cohort study of suicide in personnel who left the regular UKAF between 1996 and 2018 linking national databases of discharged personnel and suicide deaths, using survival analysis to examine the risk of suicide in veterans compared to the general population and conditional logistic regression to investigate factors most strongly associated with suicide after discharge. The 458,058 individuals who left the UKAF accumulated over 5,852,100 person years at risk, with a median length of follow–up of 13 years, were mostly male (91%), and had a median age of 26 years at discharge. 1,086 (0.2%) died by suicide. The overall rate of suicide in veterans was slightly lower than the general population (standardised mortality ratio, SMR [95% confidence interval, CI] 94 [88 to 99]). However, suicide risk was 2 to 3 times higher in male and female veterans aged under 25 years than in the same age groups in the general population (age–specific mortality ratios ranging from 160 to 409). Male veterans aged 35 years and older were at reduced risk of suicide (age–specific mortality ratios 47 to 80). Male sex, Army service, discharge between the ages of 16 and 34 years, being untrained on discharge, and length of service under 10 years were associated with higher suicide risk. Factors associated with reduced risk included being married, a higher rank, and deployment on combat operations. The rate of contact with specialist NHS mental health services (273/1,086, 25%) was lowest in the youngest age groups (10% for 16– to 19–year–olds; 23% for 20– to 24–year–olds). Study limitations include the fact that information on veterans was obtained from administrative databases and the role of pre–service vulnerabilities and other factors that may have influenced later suicide risk could not be explored. In addition, information on contact with support services was only available for veterans in contact with specialist NHS mental health services and not for those in contact with other health and social care services. Conclusions: In this study, we found suicide risk in personnel leaving the UKAF was not high but there are important differences according to age, with higher risk in young men and women. We found a number of factors which elevated the risk of suicide but deployment was associated with lower risk. The focus should be on improving and maintaining access to mental health care and social support for young service leavers, as well as implementing general suicide prevention measures for all veterans regardless of age. Cathryn Rodway and co-workers study risk of suicide in people who served in the United Kingdom armed forces. Author summary: Why was this study done?: There are few international studies investigating suicide in veterans and no recent UK–wide studies—we previously published findings from a 10–year (1996 to 2005) study of suicide in almost 234,000 people who left the UK Armed Forces (UKAF). Since this publication, the nature of serving in the UK military has changed. Periods of intense activity in Iraq and Afghanistan have resulted in concern about the mental health impact of service. The number of personnel in the Army has decreased. There are also more services available for veterans experiencing poor mental health. Patterns of suicide in the general population have changed. What did the researchers do and find?: We linked national databases of people who left the UKAF (collected by the Ministry of Defence) and people who died by suicide (collected by the National Confidential Inquiry into Suicide and Safety in Mental Health) to investigate the rate, timing, and risk factors for suicide in veterans. Between 1996 and 2018, 458,058 people left the UKAF and 1,086 (0.2%) died by suicide. The findings indicate that the overall suicide rate in veterans was similar to the general population (if anything, it was slightly lower), although the risk of suicide in men and women aged under 25 years who had left the military was 2 to 3 times higher than the same age group in the general population. Risk factors for suicide in veterans of the UKAF included being male, serving in the Army, being discharged from the military between the ages of 16 and 34 years, being untrained, and having a length of service of under 10 years. A quarter of veterans who died by suicide had been in contact with mental health services in the year before they died. What do these findings mean?: The findings from this study indicate that the overall risk of suicide in veterans is not high but that young men and women leaving the UKAF are at higher risk, especially if they have had a short length of service. Improving access to mental health care and social support for people who have left the UKAF, particularly if they are young, is important for suicide prevention in this group and may need to be maintained in the long–term. Given the number of older veterans who die by suicide, general suicide prevention measures are also needed for all veterans regardless of age. We were unable to explore the role of pre–service or other factors that may have influenced late suicide risk, such as childhood trauma, unemployment, or homelessness—these factors are important in this population and need further investigation. [ABSTRACT FROM AUTHOR]