IMPORTANCE: The timing of sexual partnerships is important for sexually transmitted infection (STI) transmission potential. Studies often measure timing as whether partnerships overlap in time (concurrency), but this measure does not account for how STI risk from previous partners can be carried forward into future partnerships even when there is a time gap between them (serial monogamy) if the infectious period is greater than this time gap. OBJECTIVE: To examine the association of the timing of partnerships, measured as the time gap or time overlap between partners, and perceptions of partners' concurrency with STI transmission. DESIGN, SETTING, AND PARTICIPANTS: This survey study that was conducted in 2017 included 8867 participants in Britain aged 16 to 44 years who reported 1 or more sexual partners in the 5 years before the interview. Data were collected from 2010 to 2012 from Britain's third National Survey of Sexual Attitudes and Lifestyles (Natsal-3), a large probability survey (response rate, 57.7%) designed to be broadly representative of the general population. EXPOSURES: Gaps between participants' 3 or fewer most recent partners in the past 5 years were calculated from dates of the last sexual encounter with former partners and the first sexual encounter with subsequent partners. Negative gaps denote overlapping partnerships (concurrency); positive gaps denote serial monogamy. Participant perception of most recent partner concurrency was proxied by asking participants whether they knew or thought that their partners had had sex with other partners since their first sexual encounter together. MAIN OUTCOMES AND MEASURES: Reported STI diagnosis in the past 5 years. RESULTS: Of 8867 participants eligible for this analysis, 3509 (39.6%) were male and 5158 (58.2%) were female, with a mean age of 28 years. Overall, 48.1% of males and 39.5% of females reported 2 or more partners and 1 or more time gaps. The median time gap was 2 months (interquartile range, -3 months to 8 months). Although 67.0% of the time gaps were 1 month or more, many were sufficiently short time gaps for STI transmission. The time gap was independently associated with STI diagnosis, without a significant decrease in likelihood until the time gap was 4 months or more for females (adjusted odds ratio [OR]: 0.39, 95% CI, 0.19-0.81) and 6 months or more for males (adjusted OR: 0.42, 95% CI, 0.20-0.85) compared with time overlaps of 2 years or more. Participant perception of partners' concurrency (reported by half of the participants) was independently associated with STI diagnosis among females (reporting no partner concurrency vs reporting partner concurrency: adjusted OR, 0.32; 95% CI, 0.22-0.49). CONCLUSIONS AND RELEVANCE: The findings suggest that the gap between partners is often sufficiently small to permit STI transmission and that many people, although themselves monogamous, have partners who are not, which itself is associated with an increase in the risk of STI acquisition. Public health practitioners should communicate these epidemiological facts, and researchers should develop measures that better capture the risk of STI transmission from partners.