I've always had some sort of inclination that the sages must have been the ones that created it. I'm pretty sure there aren't any sages until Ocarina of Time, though, so it might be better to say that it was Din, Nayru, or Farore, or all three of them.
I know that The Minish Cap explains that the Minish gave the Four Sword ("Picori Sword") to the hero in the intro, so the Master Sword might have similar origins. Some other race (like the Sheikah, perhaps) could have forged it.
There's really not an official answer until it's explained in a Zelda game, if that ever even happens.