San Martino Chapel, Medieval chapel in Lower Basilica of San Francesco, Assisi, Italy

San Martino Chapel, the first chapel on the left in the Lower Basilica of San Francesco, is a rectangular space with a vaulted ceiling and a vertical stone column with a carved capital dividing its walls. The side walls carry ten frescoes of the life of Saint Martin, read from bottom to top starting at the lower left, in muted browns, reds, blues and pale flesh tones.

Cardinal Gentile Portino da Montefiore, titular of Santi Silvestro e Martino ai Monti in Rome, funded the chapel with 600 golden florins, recorded in a document of March 1312. He likely agreed with Simone Martini in Siena that spring while moving the papal treasure to Avignon, and died at Lucca that October. Martini painted the frescoes in at least three phases between 1312 and 1318, interrupted by work in Siena and Naples.

The chapel owes its dedication to Cardinal Gentile Portino da Montefiore, titular of the Roman basilica of Santi Silvestro e Martino ai Monti. Above the entrance arch he is shown as an elderly cardinal who has left his galero on the balustrade to kneel before Saint Martin. The three rows of busts in the window reveals—knight saints, bishop or pope saints, hermit saints—follow Martin's path from soldier to Bishop of Tours to founder of monasteries.

The chapel is the first chapel on the left of the Lower Basilica of San Francesco, entered from the nave, so it is seen during a visit to the whole church. Its three windows hold stained glass made by Giovanni di Bonino in 1312–1317, probably on a design by Simone Martini.

The eight full-length saints under the entrance arch were partly repainted after Martini returned from Naples in 1317: Saints Clare, Elizabeth of Hungary and Louis of Toulouse replaced Ursula, Nicholas of Bari and Anthony of Padua, a homage to the Angevin dynasty's family saints.