Across a body of work that includes the ceiling of the Sistine Chapel and the David, exactly one sculpture bears Michelangelo's signature, and the story of how it got there is, by most accounts of the episode, one of wounded pride rather than settled artistic conviction. Shortly after the Pieta was unveiled, still in his early twenties and largely unknown outside Florence, Michelangelo reportedly overheard a group of visitors from Lombardy attributing the work to a different, better-established sculptor from their own region.
According to the traditional account, he returned that same night with a chisel and carved his name directly into the sash that runs diagonally across the Virgin's chest: "MICHAEL ANGELUS BONAROTUS FLORENTINUS FACIEBAT," Michelangelo Buonarroti of Florence made this. It remains legible on the sculpture today, cut into the marble rather than added in any less permanent way.
Michelangelo is said to have regretted the outburst almost immediately and to have vowed never to sign another work again, a vow that, so far as the surviving record shows, he actually kept for the rest of his long career. Whether the anecdote is preserved with full accuracy or has been polished somewhat in the retelling over five centuries, the physical fact of the signature itself is not in dispute: it is there, on the sash, the only place in his entire output where Michelangelo ever felt the need to say a work was unmistakably his.