Giuseppe Piermarini (18 July 1734 – 18 February 1808) was an Italian architect who trained with Luigi Vanvitelli at Rome and designed the Teatro alla Scala, Milan (1776–78), which remains the work by which he is remembered. Indeed, "il Piermarini" serves as an occasional euphemism for the celebrated opera house. Piermarini was appointed professor in the Academy of Fine Arts of Brera, better known as Brera Academy, Milan, when it was formally founded in 1776.

Piermarini was born at Foligno, then part of the Papal States. He was Vanvitelli's collaborator at Caserta, 1765-1769, when he removed to Milan. From his Milan base he reorganized the University of Pavia (1770) and the palazzo for the Accademia di Scienze e Belle Lettere, or Accademia Virgiliana, at Mantua (1773). He designed the interior of the Maria Teresa Hall of the Biblioteca Braidense in Milan

With the Habsburg decision permanently to install an archduke at Milan, Piermarini was commissioned to reconstruct the ducal palace adjoining the cathedral as an appropriate city residence and to construct a wholly new country seat near Monza.

For the archducal residence in Milan, Piermarini successfully avoided competition with the rich Gothic front of the cathedral with his sober neoclassical façade (1773–80) and created the Piazzetta Reale, as part of his urbanistic projects in the city centre. For the Royal Villa of Monza, (1776 onwards), successive changes adapted the original pleasure villa to a seat of court. In 1779 Piermarini was officially named architect of the Imperial Kingdom, a position he had occupied in fact for several years.