Gaius Sempronius Gracchus (c. 154 BC – 121 BC) was a reformist Roman politician and soldier who lived during the 2nd century BC. He is best known for his plebeian tribunates in 123 and 122 BC, during which he proposed a wide set of laws, including laws to establish colonies outside of Italy, engage in further land reform, reform the judicial system and system for provincial assignments, and create a subsidised grain supply for Rome.

The year after his tribunate of 122 BC, he and his political allies were implicated in political unrest and the armed occupation of the Aventine Hill at Rome. His political enemies used this unrest to declare martial law and march on his supporters, leading to his death either by suicide or in battle. After his death, his political allies were convicted in a series of trials, but most of his legislation was undisturbed.

His brother was the reformer Tiberius Sempronius Gracchus who was plebeian tribune in 133 BC and was also killed in political unrest that year. They were the sons of the Gracchus who was consul in 177 and 163 BC. The two brothers, known together as the Gracchi brothers, are often associated due to Gaius' own political self-representation and its transmission to contemporary times in Plutarch's Parallel Lives. The extent to which the two brothers should be associated and viewed as successors is disputed by modern scholars.

Gaius Gracchus was born into a very well-connected political family. His father, Tiberius Sempronius Gracchus, was a very successful politician of the 2nd century BC: he served in the consulships for 177 and 163 BC, and was elected censor in 169. He also had celebrated two triumphs during the 170s, one for the victorious establishment of a twenty-year-long peace in Spain.

His mother was Cornelia, daughter of Scipio Africanus, a noble woman who was a major influence on the Gracchi. As a widow, after the elder Gracchus' death, she refused the marriage proposal of Ptolemy VIII, the king of Egypt, preferring to devote her life to the upbringing of her sons.

Tiberius Gracchus, Gaius' elder brother, had through his marriage aligned the family with the Claudii Pulchri, despite his maternal connections to the Cornelii Scipiones, as the two families had a historic rivalry. It can be supposed, however, that both the Gracchi brothers would have come into contact with powerful members of both families.^{[citation needed]}

Gaius Gracchus served in the Roman army under Scipio Aemilianus during the final campaign in the Numantine War in 133 BC. He may have held the military tribunate during his service there. During his elder brother Tiberius' tribunate, he started his political career with election as a commissioner in the Gracchan land commission to distribute public land to poor families.

In 126 BC, he supported Marcus Fulvius Flaccus' political programme. While in Rome, he opposed a proposed tribunician law to prohibit non-citizens from settling in Roman towns and evict those who had done so. He then was elected as quaestor and assigned to the Roman province of Sardinia to fight the rebels there under consul Lucius Aurelius Orestes.