Can you use pokemon like regirock or dialga in pokemon official tournaments?

Question

I'm planning to enter a tournament. I have a team of legendaries, and I'm not sure if I'm allowed to enter them in the tournament. When I watch a tournament on the internet no one uses legendaries.

Are they generally allowed?

Answer 1

The rules in official tournaments can differ from season to season, but they always mention what Pokemon are not eligible for participating.

Coming back to your question, there is no definite answer. Check out the rules for the tournament you're planning to enter before thinking about what team you will be using.

Answer 2

In general, most legendaries are banned. As Jerry said, the rules can differ, but according to Bulbapedia, most if not all of the legendaries that would otherwise be legal under the rules are banned. Interestingly, although legendaries are banned, non-legendaries that are generally considered more useful/powerful (EX: Excadrill in Gen V) are allowed in.

For an explanation of what I mean: Excadrill and Blaziken are considered "Ubers" in the BW competitive community, but are permitted in tournaments. (Basically, Ubers is the banlist. Most people play "OU" (OverUsed), which forbids all Ubers pokemon. There exist tiers below OU as well, and you are only permitted pokemon from your tier or below.)

However, Jirachi, Celebi, and Kyurem-Black are legendaries banned at tournaments, but are only considered OU by the same community.

For a more modern example: Kangaskahnite (the mega evolution item for Kangaskahn) is classed as Ubers exclusive, but is allowed in Battle Spot rated battles. Presumably, it will be permitted in tournaments. The same is true for Gengarite.