Othello is an adaptation of the Italian writer Cinthio's tale "Un Capitano Moro" ("A Moorish Captain") from his Gli Hecatommithi (1565), a collection of one hundred tales in the style of Giovanni Boccaccio's Decameron. No English translation of Cinthio was available in Shakespeare's lifetime, and verbal echoes in Othello are closer to the Italian original than to Gabriel Chappuy's 1584 French translation. Cinthio's tale may have been based on an actual incident occurring in Venice about 1508. It also resembles an incident described in the earlier tale of "The Three Apples", one of the stories narrated in the One Thousand and One Nights (Arabian Nights). Desdemona is the only named character in Cinthio's tale, with his few other characters identified only as the "Moor", the "Squadron Leader", the "Ensign", and the "Ensign's Wife" (corresponding to the play's Othello, Cassio, Iago and Emilia). Cinthio drew a moral (which he placed in the mouth of Desdemona) that it is unwise for European women to marry the temperamental men of other nations. Cinthio's tale has been described as a "partly racist warning" about the dangers of miscegenation.
While Shakespeare closely followed Cinthio's tale in composing Othello, he departed from it in some details. Brabantio, Roderigo, and several minor characters are not found in Cinthio, for example, and Shakespeare's Emilia takes part in the handkerchief mischief while her counterpart in Cinthio does not. Unlike in Othello, in Cinthio, the "Ensign" (the play's Iago) lusts after Desdemona and is spurred to revenge when she rejects him. Shakespeare's opening scenes are unique to his tragedy, as is the tender scene between Emilia and Desdemona as the lady prepares for bed. Shakespeare's most striking departure from Cinthio is the manner of his heroine's death. In Shakespeare, Othello suffocates Desdemona, but in Cinthio, the "Moor" commissions the "Ensign" to bludgeon his wife to death with a sand-filled stocking. Cinthio describes each gruesome blow, and, when the lady is dead, the "Ensign" and the "Moor" place her lifeless body upon her bed, smash her skull, and cause the cracked ceiling above the bed to collapse upon her, giving the impression its falling rafters caused her death. In Cinthio, the two murderers escape detection. The "Moor" then misses Desdemona greatly, and comes to loathe the sight of the "Ensign". He demotes him, and refuses to have him in his company. The "Ensign" then seeks revenge by disclosing to the "Squadron Leader" the "Moor's" involvement in Desdemona's death. The two depart Cyprus for Venice, and denounce the "Moor" to the Venetian Seignory; he is arrested, taken to Venice, and tortured. He refuses to admit his guilt and is condemned to exile. Desdemona's relatives eventually find and kill him. The "Ensign", however, continues to escape detection in Desdemona's death, but engages in other crimes while in Venice. He is arrested and dies after being tortured. Cinthio's "Ensign's Wife" (the play's Emilia), survives her husband's death to tell her story.
Cinthio's "Moor" is the model for Shakespeare's Othello, but some researchers believe the poet also took inspiration from the several Moorish delegations from Morocco to Elizabethan England circa 1600.
Another possible source was the Description of Africa by Leo Africanus. The book was an enormous success in Europe, and was translated into many other languages, remaining a definitive reference work for decades (and to some degree, centuries) afterwards. An English translation by John Pory appeared in 1600 under the title A Geographical Historie of Africa, Written in Arabicke and Italian by Iohn Leo a More... in which form Shakespeare may have seen it and reworked hints in creating the character of Othello.
While supplying the source of the plot, the book offered nothing of the sense of place of Venice or Cyprus. For knowledge of this, Shakespeare may have used Gasparo Contarini's The Commonwealth and Government of Venice, in Lewes Lewkenor's 1599 translation.