Poker is widely considered a sport, and it is officially considered a "mind sport", along with other games, such as chess. There actually is a mind sports Olympics, called the Mind Sports Olympiad. There is also what seems to be the more prestigious event, the World Mind Sports Games. Poker events are held at the former and it looks like they will eventually be included in the latter. The WMSG is interesting, because its creator specifically envisions a third type of Olympic Games, along with the Summer and Winter Games.
I would like to elaborate on vwls' post.
Yes, many people consider poker a sport but I personally do not. I also don't consider chess a sport.
Poker is a game, a competitive game to be exact. Not all games are sports and not all sports are games.
The dictionary defines "sport" as the following:
Sport - an activity involving physical exertion and skill in which an individual or team competes against another or others for entertainment.
By the literal definition, poker is clearly not a sport. You simply do not physically exert yourself.
But, just to entertain the discussion - let's say "sport" did not require physical exertion.
Again, poker is a game - not a sport. If you consider poker a sport then you would also have to classify Magic The Gathering, Pokemon, Yugioh, video game tournaments, competitive checkers, etc as sports as well.....
We run into a problem trying to determine whether poker is a sport or not because it is a game of skill and it is also mentally taxing.
For me, when it comes down to it.... poker is a competitive game of skill that exhausts mental effort. It is, unarguably a highly competitive game. But it is just that - game. Not a sport, in my opinion.