It's not unprofessional, but you just have to be careful not to go OTT and at least get one in a place where it's easy to cover up if necessary, because as easy as it is for us to say it's fine, other people have a different view - do you really want that hindering you with future job prospects as an example?
EDIT: As an example, the head of my site (a good engineer) is covered in tattoos and earn God knows how many hundred thousands per year. So it's not that much of a hindrance to some.