Valeria Messalina (17–48) was the third wife of the Roman emperor Claudius. She was the daughter of Domitia Lepida and Marcus Valerius Messalla Barbatus.

Her father was the son of Marcus Valerius Messalla Messallinus and Claudia Marcella Minor, daughter to Gaius Claudius Marcellus and Octavia. Her mother was the granddaughter of Mark Antony and Octavia through their daughter Antonia Major. As grandchildren of Octavia, her parents were first cousins. Octavia herself was sister to Caesar Augustus. Thus Messalina was closely related to the Julio-Claudian dynasty and had been a regular at the court of Caligula.

She married Claudius around 38 and bore him two children, Britannicus (41–55), who might actually have been fathered by Caligula, and Octavia (40–62), who married her own stepbrother, the emperor Nero. In 48, Messalina conspired with Caius Silius to kill Claudius while her husband was in Ostia. She actually went through a public marriage ceremony with Silius (he was already married to Junia Silana). Apparently, she was motivated by the protection the powerful and popular Silius could give her over the weakness of Claudius. Her plotting was sufficiently promising that many senior officials were swayed to her side. However, the plot was exposed by Narcissus, an advisor to Claudius. Messalina, Silius and a number of others were summarily executed. Messalina was apparently offered the opportunity of suicide but was unable to do it. (Claudius was at dinner when he was informed of her death; his response was to ask for more wine.)

Her reputation is very poor. A number of Roman historians (mainly Tacitus and Suetonius) portray her as a cruel, avaricious, foolish nymphomaniac who had many wild affairs and once challenged a notorious Roman prostitute named Scylla to an all-night sex competition. (Scylla gave up at dawn, but Messalina continued well into the morning.) She duped Claudius and manipulated him into executing those who displeased or spurned her. She is also recorded as a lover of parties and festivities as well as an enthusiastic player of court politics who sold her influence to Roman nobles and foreign notables. Her name is now used as a synonym for others with her supposed vices.

As a wife, she succeeded Plautia Urgulanilla and Aelia Paetina. She was in turn replaced by Agrippina the Younger.