Ambrogio Lorenzetti (or Ambruogio Laurati; c. 1290 – June 9, 1348) was an Italian painter of the Sienese school. He was active between approximately from 1317 to 1348. His elder brother was the painter Pietro Lorenzetti.
His work shows the influence of Simone Martini, although more naturalistic. The earliest dated work of the Sienese painter is a Madonna and Child (1319, Museo Diocesano, San Casciano). His presence was documented in Florentine up until 1321. He would return there after spending a number of years in Siena.
The frescoes on the walls of the Hall of the Nine (Sala dei Nove) or Hall of the Peace (Sala della Pace) in the Palazzo Pubblico of Siena are one of the masterworks of early renaissance secular painting. The "nine" was the oligarchal assembly of guild and monetary interests that governed the republic. Three walls are painted with frescoes consisting of a large assembly of allegorical figures of virtues in the Allegory of Good Government. In the other two facing panels, Ambrogio weaves panoramic visions of Effects of Good Government on Town and Country, and Allegory of Bad Government and its Effects on Town and Country (also called "Ill-governed Town and Country"). The better preserved "well-governed town and country" is an unrivaled pictorial encyclopedia of incidents in a peaceful medieval "borgo" and coutryside.
The first evidence of the existence of the hourglass can be found in one of his paintings.
Like his brother, he is believed to have died of bubonic plague 1348. Giorgio Vasari includes a biography of Lorenzetti in his Lives.