Pietro de 'Rossi was an Italian nobleman, belonging to the Rossi family. Son of Bertrando and Eleonora Rossi, in 1396 he succeeded his father as count of San Secondo, Berceto and Corniglio. In order to more easily enjoy the possession of San Secondo, he obtained in the early 1400s from the bishop Giovanni Rusconi, confirming the purchase and in the following year (24 October 1401) he bought the neighboring feud of the Pizzo from the canons in Parma. exclusion of the villas of San Quirico, Ronchetti, Corticelli and Castellaicardi, which however, over time, all passed under the signoria rossiana.
In 1404 he shared the dominion of Parma with Ottobuono de 'Terzi, but the following year he was forced to abandon it after Ottobuono began terrible persecutions against his family.
Three years later, in 1407, he had to tame a revolt in San Secondo due to the burden of taxes. The rebels barricaded themselves, resisting for some months the siege set by Rossi and his army. Asked for the surrender, they obtained it by making an act of obedience, but the fortified village suffered considerable damage and remained for a long time in complete abandonment.
After 1408 he was taken prisoner by the Fieschi, against whom he had entered into conflict for the possession of Pontremoli. After being released he passed to the court of the Estensi, new lords of Parma. In 1413 he obtained the investiture on San Secondo by the emperor Sigismondo. In 1413, following the marquis Niccolò d'Este, he went to Jerusalem, where he was decorated as a sign of honor and distinction of the cingulum of the Holy Sepulcher.

However, the changed defensive requirements led to privilege the village of San Secondo to the administrative heart of Rossio that was until then Felino, so Pier Maria began to rebuild and fortify the village of San Secondo which was surrounded by walls and surrounded by a moat that it was filled with the waters coming from the dead taro, then building, on the opposite bank, the Rocca dei Rossi around 1413. In 1420 it ceded Parma to the Visconti family and retired to Venice.
He was a theorist and composer of music. Giorgio Anselmi in the treatise De harmonia dialogi indicates him as an illustrious teacher, dedicating his work to him. He composed three three-part motets and a two-part ballad.