Background Transactional sex may increase risk of HIV and sexually transmissible infections (STIs). In Laos, men who have sex with men are disproportionately affected by HIV, and bisexual behaviour among men is relatively common. The occurrence of transactional sex among behaviourally bisexual men in Vientiane, Laos was explored. Methods: In 2010, behaviourally bisexual men were recruited through enhanced snowball sampling to complete a behavioural survey. Reports of transactional sex partners (anal/vaginal sex) in the previous year, by direction of payment and partner gender, is described. Results: Of 88 participating behaviourally bisexual men (median age 22 years), 17 (19 ) reported only selling sex, eight (9 ) reported only paying for sex and nine (10 ) reported both selling and paying for sex. Men reporting any transactional sex reported a median of four transactional sex partners and reported a higher number of total sex partners in the previous 12 months (median: 18.5 partners) than men reporting no transactional sex partners (median: 6 partners). Of 26 men who reported selling sex, 15 (58 ) were paid by females, 15 (58 ) were paid by males and 14 (55 ) were paid by transgender sex partner(s); 11 (42 ) reported consistent condom use (CCU) when selling sex. Of 17 men who reported paying for sex, 13 (76 ) paid females, six (35 ) paid males and two (12 ) paid transgender partner(s); eight (47 ) reported CCU when paying for sex. Conclusions: Young behaviourally bisexual men engaging in transactional sex may be at increased risk of HIV and STIs. Prevention interventions should consider the transient and informal nature of transactional sex in this population.