Whether it is legal or not to play roulette or any other casino game, entirely depends on your country of residence and the online casino of your choice. There is no universal gambling law that applies to all countries as each one has its own regulations and policies when it comes to online gambling.
For instance, the UK is a good example of a country in which online gambling is permitted. In fact, this type of gambling activity has become a major source of entertainment for many people as all online casinos that operate on this territory are regulated by the UK Gambling Commission and other regulatory bodies.