Ambrogio Lorenzetti (or Ambruogio Laurati) (c. 1290 - 1348) was a Sienese painter, he was born and died in Siena. He was active between approximately 1317 and 1348. His brother was the painter Pietro Lorenzetti. His work shows the influence of Simone Martini, although it is more naturalistic.

Ambrogio's works include

• Virgin and Child Enthroned (1319)
• San Procolo altarpiece (1332)
• Franciscan Martyrdom at Bombay (c.1336), fresco at San Francesco, Siena
• Frescoes at the Palazzo Pubblico in Siena. These frescoes are composed of Effects of Good Government on Town and Country, Allegory of Good Government [1] and Allegory of Bad Government and its Effects on Town and Country
• Santa Petronilla Altarpiece (1340s)

We find the first evidence of the existence of the hourglass on one of his paintings (see external link above). He is believed to have died of bubonic plague circa 1348/9.

Giorgio Vasari includes a biography of Lorenzetti in his Lives.