Strangely enough, there are two different answers, both correct. The first would be Saint Sebastian (...who is the official saint of athletes). The second being Saint Rita.
...She became the unofficial Patron Saint of baseball, after the story of a group of nuns who were conned into investing in dry oil well in Texas during the early 1920's. The nuns prayed to Saint Rita, because she was the patron saint of impossible and desperate causes, and the well miraculously started pouring out oil. Apparently the workers who built the well for the nuns, also built a baseball diamond for recreation (some of whom got good enough to play professionally). So, Saint Rita was now loosely associated to baseball.
Being a huge baseball fan I was curious about this. Reading further in the article it references how the movie "The Rookie" certainly had some influence over the idea of St Rita being the patron saint of baseball. The article does state however that she is the official saint of impossible causes so that would fit the bill for my Minnesota Twins :-) In all seriousness though, it would appear that Saint Sebastian would be the official answer for now. Maybe some baseball player will be declared a saint someday and then that person can be the official patron saint of baseball