Objective: To assess the correlates of the high risk sexual behaviors of Polish migrants in the United Kingdom (UK) after 2004, and to compare such behaviors before/after immigration. Methods: In 2013, a cross-sectional study was conducted through the use of a Computer-assisted web interviewing surveying technique with the use of a self-administered questionnaire. Results: Among 408 respondents (56.9% women), with a median age of 32 years, significantly more admitted to having unprotected sexual contact with a casual partner while in the UK (p < 0.0001) than while in Poland; more were engaged in sex after the use of recreational drugs and alcohol (p < 0.0001 and p = 0.001 respectively). Being a male was associated with greater odds of unprotected sex, sex after the use of alcohol, and having multiple partners. Being single and having only been a resident for a short time in the UK, presenting a lower self-esteem, were predictors of unprotected sex. A total of 19.6% of the respondents admitted to having been tested while in Poland, a lower (p < 0.0001) frequency than while in the UK (49.5%); this referred to both genders; 1.2% (95% CI: 0.79–2.83%) reported that they were HIV positive. Conclusions: Migration can create a vulnerability to STIs, especially for single male migrants with low self-esteem, staying in the UK for less than two years. The results point to strengthening strategies which help reduce high risk sexual behavior among Polish migrants, and to introduce interventions to promote an awareness of HIV sero-status.