Italian Humanist and founder of the Academia Romana, a semisecret society devoted to archaeological and antiquarian interests and the celebration of ancient Roman rites.

As a youth, Laetus decided to dedicate his life to the study of the ancient world. He went to Rome c. 1450 and, in 1457, succeeded Lorenzo Valla, his former teacher, as professor of eloquence in the Gymnasium Romanum. From the outset he gathered round him a number of Humanists in a semisecret society, the Academia Romana. The members, who changed their Christian names to pagan ones, met not only to discuss their antiquarian and archaeological interests but to celebrate, under the direction of Laetus as pontifex maximus, rites and mysteries of pagan Rome, such as the birth of Romulus and the festival of the Palilia. Their admiration for the ancient world thus developed into a materialistic vision of life, in conscious opposition to Christian ideals, with the possible object of achieving revolutionary political reforms. Hence the Academia Romana fell under the suspicion of Pope Paul II, who in February 1468 dissolved it and arrested its members. Laetus, who for about a year had been living in Venice, was brought to Rome and imprisoned, but by May 1469 he and many of his associates were set free by an act of clemency. They did not regain full liberty until Sixtus IV, who succeeded Paul II in 1471, not only restored Laetus' professorship but also allowed the reconstitution of the Academia Romana.

Between his release and his death Laetus pursued his scholastic studies with extraordinary zeal, interrupted only by two visits to northern Europe (1472–73, 1479–83). His works of this period include treatises concerning Roman antiquities, commentaries on Latin authors, and, most important, some editiones principes among which are those of Curtius and Varro, Pliny's Letters, and Sallust. Laetus is not regarded highly as a Humanist: despite his erudition, the lack of rigour and absence of critical spirit in his method cause his philological achievements to be treated with reserve by modern scholars.