The details of Boccaccio's birth are uncertain. He was born in Florence or in a village near Certaldo where his family was from. He was the son of Florentine merchant Boccaccino di Chellino and an unknown woman; he was likely born out of wedlock. Boccaccio's stepmother was called Margherita de' Mardoli.
Boccaccio grew up in Florence. His father worked for the Compagnia dei Bardi and, in the 1320s, married Margherita dei Mardoli, who was of a well-to-do family. Boccaccio may have been tutored by Giovanni Mazzuoli and received from him an early introduction to the works of Dante. In 1326, his father was appointed head of a bank and moved with his family to Naples. Boccaccio was an apprentice at the bank but disliked the banking profession. He persuaded his father to let him study law at the Studium (the present-day University of Naples), where he studied canon law for the next six years. He also pursued his interest in scientific and literary studies.
His father introduced him to the Neapolitan nobility and the French-influenced court of Robert the Wise (the king of Naples) in the 1330s. At this time, he fell in love with a married daughter of the king, who is portrayed as "Fiammetta" in many of Boccaccio's prose romances, including Il Filocolo (1338). Boccaccio became a friend of fellow Florentine Niccolò Acciaioli, and benefited from his influence as the administrator, and perhaps the lover, of Catherine of Valois-Courtenay, widow of Philip I of Taranto. Acciaioli later became counselor to Queen Joanna I of Naples and, eventually, her Grand Seneschal.
It seems that Boccaccio enjoyed law no more than banking, but his studies allowed him the opportunity to study widely and make good contacts with fellow scholars. His early influences included Paolo da Perugia (a curator and author of a collection of myths called the Collectiones), humanists Barbato da Sulmona and Giovanni Barrili, and theologian Dionigi di Borgo San Sepolcro.
In Naples, Boccaccio began what he considered his true vocation of poetry. Works produced in this period include Il Filostrato and Teseida (the sources for Chaucer's Troilus and Criseyde and The Knight's Tale, respectively), The Filocolo (a prose version of an existing French romance), and La caccia di Diana (a poem in terza rima listing Neapolitan women). The period featured considerable formal innovation, including possibly the introduction of the Sicilian octave, where it influenced Petrarch.