most arduinos come with a 7805 voltage regulator, which 12 volts is fine, It will become hot without a heatsink so keep an eye on it
BUT there are some boards (the modern device barebones for example) that come with a tiny LDO regulator, which 12v would really be pushing its max
see what you have, just to make sure, and try and figure out how much current your going to be drawing tru it (a 7805 should be ok @ 12v with a really low current draw, its fine for me with a tiny heatsink up till near 500ma with a 12v supply)