Background and aims: Online gambling participation is increasing rapidly, with relatively little research about the possible effects of different gambling activities on problem gambling behaviour. The aim of this exploratory study was to examine the participation in online gambling activities and the relationship with problem gambling among an international sample of online gamblers. Methods: An online gambling survey was posted on 32 international gambling websites and resulted in 1,119 respondents over a four-month period. Results: Poker was the most popular gambling activity online. A number of online activities were associated with problem gambling, including: roulette, poker, horse race betting, sports betting, spread betting and fruit (slot) machines. Not surprisingly, those that gambled on these activities regularly (except poker) were more likely to be a problem gambler, however, what is interesting is that the reverse is true for poker players; those that gambled regularly on poker were less likely to be a problem gambler compared to the non-regular poker players. The majority of the players also gambled offline, but there was no relationship between problem gambling and whether or not a person also gambled offline. Discussion: Problem gambling is associated more with certain online gambling activities than others, and those gambling on two or more activities online were more likely to be a problem gambler. Conclusion: This paper can help explain the impact different online gambling activities may have on gambling behaviour. Consideration needs to be given to the gambling activity when developing and implementing treatment programmes.