Background: The vulnerability of women to HIV infection makes establishing predictors of women's involvement in extra-marital partnerships critical. We investigated the predictors of extra-marital partnerships among women married to fishermen. Methods: The current analyses are part of a mixed methods cross-sectional survey of 1090 gender-matched interviews with 545 couples and 12 focus group discussions (FGDs) with 59 couples. Using a proportional to size simple random sample of fishermen as our index participants, we asked them to enrol in the study with their spouses. The consenting couples were interviewed simultaneously in separate private rooms. In addition to socio-economic and demographic data, we collected information on sexual behaviour including extra-marital sexual partnerships. We analysed these data using descriptive statistics and multivariate logistic regression. For FGDs, couples willing to participate were invited, consented and separated for simultaneous FGDs by gender-matched moderators. The resultant audiofiles were transcribed verbatim and translated into English for coding and thematic content analysis using NVivo 9. Results: The prevalence of extra-marital partnerships among women was 6.2% within a reference time of six months. Factors that were independently associated with increased likelihood of extra-marital partnerships were domestic violence (aOR, 1.45; 95% CI 1.09-1.92), women reporting being denied a preferred sex position (aOR, 3.34; 95% CI 1.26-8.84) and spouse longer erect penis (aOR, 1.34; 95% CI 1.00-1.78). Conversely, women's age - more than 24years (aOR, 0.33; 95% CI 0.14-0.78) and women's increased sexual satisfaction (aOR, 0.92; 95% CI 0.87-0.96) were associated with reduced likelihood of extra-marital partnerships. Conclusion: Domestic violence, denial of a preferred sex positions, longer erect penis, younger age and increased sexual satisfaction were the main predictors of women's involvement in extra-marital partnerships. Integration of sex education, counselling and life skills training in couple HIV prevention programs might help in risk reduction. © 2014 Kwena et al.