An elder (and also illegitimate) half-brother, Galeotto Roberto Malatesta, born in 1411, was the issue of the relationship of their father Pandolfo III with Allegra de' Mori.

Following the family's tradition, Sigismondo after the death of his father debuted as man-at-arms at the age of 13 against his relative Carlo II Malatesta, lord of Pesaro and Pope Martin V's ally, who aimed to annex Rimini, Cesena and Fano to his territories.

In 1432 he accepted the command of a papal corps, defeating the Spanish condottiero Sante Cirillo and thwarting Antonio I Ordelaffi's attempt to capture Forlì (1435–36).

In the meantime, in 1434 he married his niece Ginevra d'Este, Niccolò III's legitimate daughter by his second wife Parisina Malatesta, first cousin of Sigismondo; they had one son, Galeotto Roberto Novello, who died as an infant, in 1438.

Pope Pius II, who considered him guilty of treachery towards Siena arising from his long-running feud with Federico da Montefeltro, Duke of Urbino, excommunicated him, declaring him a heretic and attributing to Sigismondo a series of sins (incest, sodomy against his son Roberto and others) which smeared his reputation for centuries.

"[citation needed] Malatesta's image was publicly burnt in Rome, and a de facto crusade was then launched against him, in a league including the pope, the king of Naples, the Duke of Milan and Federico da Montefeltro.

According to the Catholic Encyclopedia:[3] From his childhood he was a skilful and daring soldier, and throughout his life was regarded as almost the first captain in Italy.He was not a religious man, and his Tempio Malatestiano, also known as San Francesco, built in Rimini, by Leon Battista Alberti and decorated by artists including Piero della Francesca and Agostino di Duccio, was essentially a lay monument to Isotta degli Atti, his lover and third wife.

Malatesta's reputation was largely based on Pius II's perception of him, although numerous contemporary chronicles described him as a tyrant and a womanizer: he delved into "rape, adultery, and incest".

[5] His deeds and political manoeuvers were characterized by all the play of violence, intrigues and subtleties typical of Renaissance Italy; however, Sigismondo was well aware of his sins, and tried to justify them in a series of love sonnets dedicated to Isotta.

The title of this book may be partly responsible for the notion that Sigismondo was known as "the Wolf of Rimini" by contemporaries or indeed subsequently at any point before the 21st century when this sobriquet has gained some ground.

Largely influenced by Pound, as well as by C. G. Jung, the critic Adrian Stokes devoted a study, The Stones of Rimini (1934), to the art created at Sigismondo's court.

Early in his writing career, E. M. Forster attempted a historical novel about Malatesta and Gemistus Pletho, but was not satisfied with the result and never published it - though he kept the manuscript and later showed it to Naomi Mitchison.