Background This cross-sectional study investigated sexually transmissible infection (STI) knowledge, safer sex attitudes and behaviours of sexually active, heterosexual men using an Internet dating service, and explored factors associated with protective attitudes. Methods: An anonymous online survey was sent to men who had logged onto an Internet dating service within the previous year. Factors associated with discussing STIs before sex and condom use intention with a new partner were analysed. Results: In total, 2339 responses were analysed. Men had lower STI knowledge scores, lower use of condoms and greater beliefs that condoms reduced interest in sex as age increased. Younger men (aged 18–29 years) were more likely to report intentions of only having sex with a new partner if a condom was used compared with older men aged over 60 years. Predictors of having a discussion about STIs with a new partner were older age, better knowledge of STIs, greater distress at a potential diagnosis of an STI and stronger future condom use intention. Intention to only have sex with a new female partner if a condom was used was associated with higher STI knowledge, discussing STIs with a new partner and distress at a diagnosis of an STI. Men with a higher number of sexual partners in the last year were less likely to have an STI discussion with new partners or intend to use a condom with a new partner. Conclusions: Older Internet-dating, heterosexual men are vulnerable to STIs. Health promotion interventions to increase STI awareness, condom use and STI testing in older men are warranted.