The play opens with Roderigo, a rich and dissolute gentleman, complaining to Iago, an ensign, that Iago has not told him about the secret marriage between Desdemona, the daughter of a Senator named Brabantio, and Othello, a Moorish general in the Venetian army. He is upset by this development because he loves Desdemona and had previously asked her father for her hand in marriage. Iago hates Othello for promoting a younger man named Michael Cassio above him, and tells Roderigo that he plans to use Othello for his own advantage. Iago is also angry because he believes, or at least gives the pretence of belief, that Othello slept with his wife Emilia. Iago denounces Cassio as a scholarly tactician with no real battle experience; in contrast, Iago is a battle-tested soldier. By emphasizing Roderigo's failed bid for Desdemona, and his own dissatisfaction with serving under Othello, Iago convinces Roderigo to wake Brabantio, Desdemona's father, and tell him about his daughter's elopement. Iago sneaks away to find Othello and warns him that Brabantio is coming for him.

Before Brabantio reaches Othello, news arrives in Venice that the Turks are going to attack Cyprus; therefore Othello is summoned to advise the senators. Brabantio arrives and accuses Othello of seducing Desdemona by witchcraft, but Othello defends himself successfully before an assembly that includes the Duke of Venice, Brabantio's kinsmen Lodovico and Gratiano, and various senators. He explains that Desdemona became enamored of him for the sad and compelling stories he told of his life before Venice, not because of any witchcraft. The senate is satisfied, but Brabantio leaves saying that Desdemona will betray Othello. By order of the Duke, Othello leaves Venice to command the Venetian armies against invading Turks on the island of Cyprus, accompanied by his new wife, his new lieutenant Cassio, his ensign Iago, and Iago's wife, Emilia as Desdemona's attendant.

The party arrives in Cyprus to find that a storm has destroyed the Turkish fleet. Othello orders a general celebration and leaves to spend private time with Desdemona. In his absence, Iago schemes to get Cassio drunk after Cassio's own admission that he cannot hold his wine. He then persuades Roderigo to draw Cassio into a fight. The resulting brawl alarms the citizenry, and Othello is forced to quell the disturbance. Othello blames Cassio for the disturbance and strips him of his rank. Cassio is distraught, but, as part of his plan to convince Othello that Cassio and Desdemona are having an affair, Iago persuades Cassio to importune Desdemona to act as an intermediary between himself and Othello, in order to convince her husband to reinstate him.