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Highlights • Starting cohabitation and getting married were associated with increased BMI. • Findings were mixed for the effect of starting cohabitation/marriage on PA. • Limited evidence for the effect of starting cohabitation/marriage on diet., Our aim was to systematically review the effect of cohabitation and marriage on physical activity, diet and weight-related outcomes during emerging adulthood. A systematic search of six electronic databases was conducted until July 2019 (PROSPERO:CRD42018106943). Prospective studies were included if data were presented for a weight-related outcome, physical activity, and/or diet among 15–35 years-old participants assessed pre- and post-cohabitation or marriage and compared to a consistently non-cohabiting/non-married (single) reference group. Following title/abstract screening, two reviewers independently screened full-text and assessed risk of bias. There were 11 studies that met inclusion criteria. Outcomes included: body mass index (BMI) only (n = 3), physical activity only (n = 4), diet only (n = 2), BMI and physical activity (n = 1), and all outcomes (n = 1). Cohabitation or marriage was associated with greater BMI increases compared to remaining single among both men and women. Three studies analysed separately cohabitation and marriage and 3 of 4 found that only marriage was associated with higher BMI. Compared to being consistently single, starting cohabitation or getting married were associated with decreased physical activity in 2 of 4 studies among men and 4 of 6 studies among women, with no differences between marriage and cohabitation. Of the three studies examining change in diet, two showed no difference between individuals beginning to cohabit compared to those remaining single; without gender differences. Starting cohabitation and getting married may be valuable targets for weight management interventions, but more studies are needed to investigate the effect of cohabitation and marriage on health behaviours.