The paper reassesses the late career of Taddeo di Bartolo (d. 1422), the most illustrious Sienese painter of the time. By close analysis of a damaged and hardly known fresco formerly in an external tabernacle at Santa Maria in Trastevere, the author argues it to be Taddeo's work about 1420. The attribution confirms, modifies, and expands a tentative suggestion of Serena Romano. The revelation that Taddeo was in Rome substantially reconfigures his last years. A donor portrait and arms indicate that Taddeo's patron was the renowned cardinal and papal diplomat, Rinaldo Brancaccio (d. 1427), known to art historians for his tomb by Donatello and Michelozzo. The author traces the careers of the painter and the prelate to identify their contact, at Siena in 1407, Pisa in 1409, and Florence about 1419, before Taddeo produced the fresco at Rome. The painting was a precocious effort by one of Martin V's men to renew the desolate Eternal City. Brancaccio's other patronage is examined to reveal that Taddeo formulated a personal, probably votive iconography for the cardinal. Subsequently it was applied in two other Brancaccio painting commissions, and in his marble tomb lunette. The essay also considers the patronage of Masaccio's and Masolino's unusual double-sided polyptych for Santa Maria Maggiore, which is undocumented. Brancaccio has been proposed as its patron, in part because he was archpriest of the great basilica for many years. Recent studies instead point to Antonio Casini, Brancaccio's successor and Taddeo's countryman and bishop, as commissioner. The author reconciles these views by hypothesizing that the notion of a two-sided altarpiece began with Brancaccio. For this enterprise he called Taddeo di Bartolo, veteran painter of a double-face painting at Perugia, to Rome.