Previous studies show an association between multiple sexual partnerships and increased sexually transmitted infection (STI) diagnoses among Navy and civilian populations, in particular among those who report Black race/ethnicity. Research reports that the disproportionate burden of multiple sexual partnerships among those who report Black race/ethnicity could be associated with low sex ratio (few men compared to women). It is unknown if the same association with multiple partnerships exists when sex ratio is high (many men compared to women). In the U.S. Navy, men outnumber women 5 to 1. If unbalanced sex ratio results in a greater number of multiple sexual partnerships, STI diagnoses could increase, resulting in reduced readiness that may negatively impact the Navy's mission. The current study examines whether an association exists between high sex ratio and multiple sexual partnerships among shipboard U.S. military personnel. Data from 1,826 deployed shipboard male and female personnel were collected from 2012-2014. Assortative sex ratios (ASR) measuring the ratio of males to females who reported the same racial/ethnic background and disassortative sex ratios (DSR) measuring the number of males to females of different racial/ethnic backgrounds were calculated for each ship. Logistic regression analysis stratified by ethnicity showed a significantly lower proportion of multiple sexual partnerships on high DSR ships and among females, after adjusting for other variables and including interaction terms. The adjusted model showed those reporting Black race/ethnicity from high DSR ships were significantly less likely to report multiple sexual partnerships compared to low DSR ships (OR=0.57; 95% CI=0.39, 0.83; p=0.003). Participants reporting non-Black race/ethnicities from high DSR ships were also significantly less likely to report multiple sexual partnerships compared to low DSR ships (OR=0.72; 95% CI=0.58, 0.89; p=0.003). Women reporting Black race/ethnicity were significantly less likely than men reporting Black race/ethnicity to report multiple sexual partnerships (OR=0.35; 95% CI=0.17, 0.74; p=0.006). Women reporting non-Black race/ethnicities were significantly less likely to report multiple sexual partnerships compared to men reporting non-Black race/ethnicities (OR=0.52; 95% CI=0.36, 0.77; p=0.001). These data contrast previous findings among low sex ratio populations and suggest women and men have fewer multiple partners in higher sex ratio environments.