Michelangelo signed only one piece of art.
tldr, summary in bold
Michelangelo is one of the few people in history whose work has spanned the centuries with no need for a last name. Hundreds of his surviving works of art — including sculptures, paintings, and drawings — don’t even bear an artist’s mark. That’s because the artist only ever signed one piece, the Madonna della Pietà, and his doing so likely stemmed from misplaced credit.
Michelangelo was commissioned to sculpt the Pietà in the late 1490s. As he was just 24 years old at the time, it was one of his earliest projects, and a piece that helped launch him into the spotlight. The sculpture, which was created as a funeral monument for French Cardinal Jean de Bilhères, depicts the Virgin Mary holding the body of Jesus following the Crucifixion. The young artist sculpted the piece from one cut of marble and finished the job in under two years. According to fellow Renaissance artist and friend Giorgio Vasari, Michelangelo’s statue initially bore no indication of its creator, but the artist supposedly changed his mind after overhearing his work credited to a rival; he snuck back to chisel his name prominently onto the sash across Mary’s chest.
Shame at the rash decision likely kept Michelangelo from signing his future works of art, though the artist did find other ways of inserting his likeness into his work. In the Sistine Chapel, Michelangelo imposed his own features onto St. Bartholomew — who was skinned alive — possibly as a joke meant to share his disdain for the physically grueling project.