Guidobaldi or Guido Baldi), Marquis del Monte, was an Italian mathematician, philosopher and astronomer of the 16th century.

After serving in the army, Guidobaldo returned to his estate of (Monte Baroccio) Mombaroccio in the Marche, where he was able to spend his time doing research into mathematics, mechanics, astronomy and optics.

His invention of a drafting instrument for constructing regular polygons and dividing a line into any number of segments was incorporated as a feature of Galileo's geometric and military compass.

He then commended Galileo to his brother, the Cardinal Del Monte, who referred him to the powerful Duke of Tuscany, Ferdinando I de' Medici.

Guidobaldo wrote an influential book about perspective, titled Perspectivae Libri VI, published at Pesaro in 1600.