Born in Trieste, Rietti was influenced by the painters Eugenio Scomparini and Giuseppe Barison. Between 1882 and 1884 he lived in San Giovanni Valdarno, painting his works under the influence of the Tuscan realism. In 1884 Rietti entered the Academy in Munich. Here followed the lessons of Franz von Defregger and Nikolaus Gysis, while Hugo von Habermann introduced him to the technique of mixed pastel with tempera, which would become characteristic of his production. In 1887 he moved to Milan, becoming a successful portrait painter and pastelist and working mainly for the Milanese high classes. Rietti, in his various stays in Vienna, Paris, Milan, Rome, Trieste, cultivated the friendship and esteem of established artists such as Habermann, Troubetzkoy, Auguste Rodin, but also Arturo Toscanini, Giacomo Puccini and Gabriele D'Annunzio, the family Casati and the Bugatti. He was a great friend of Italo Svevo.