Agaja (also spelled Agadja and also known as Trudo Agaja or Trudo Audati) was a king of the Kingdom of Dahomey, in present-day Benin, who ruled from 1718 until 1740. He came to the throne after his brother King Akaba. During his reign, Dahomey expanded significantly and took control of key trade routes for the Atlantic slave trade by conquering Allada (1724) and Whydah (1727). Wars with the powerful Oyo Empire to the east of Dahomey resulted in Agaja accepting tributary status to that empire and providing yearly gifts. After this, Agaja attempted to control the new territory of the kingdom of Dahomey through militarily suppressing revolts and creating administrative and ceremonial systems. Agaja died in 1740 after another war with the Oyo Empire and his son Tegbessou became the new king. Agaja is credited with creating many of the key government structures of Dahomey, including the Yovogan and the Mehu.

The motivations of Agaja and his involvement with the slave trade remain an active dispute among historians of Dahomey with some arguing that he was resistant to the slave trade but agreed to it because of the need to defend his kingdom, while others argue that no such motivation existed and the wars against Allada and Whydah were simply for economic control.

Agaja served a crucial role in the early development of the Kingdom of Dahomey. The kingdom had been founded by Agaja's father Houegbadja who ruled from 1645 until 1685 on the Abomey plateau. Although there were some limited military operations outside of the plateau, the kingdom did not significantly expand before the eighteenth century.

Oral tradition says that Agaja was born around 1673, the second oldest son to Houegbadja. Houegbadja's first two children were the twins Akaba and Hangbe. Agaja was originally called Dosu, a traditional Fon name for the first son born after twins. When Houegbadja died, Akaba became the king and ruled from 1685 until about 1716. Akaba died during a war in the Ouémé River valley and since his oldest son, Agbo Sassa, was a minor, his twin sister Hangbe may have ruled for a brief period of time (alternatively given as either three months or three years). Hangbe supported a faction that wanted Agbo Sassa to be the next king, but Agaja contested this and became the ruler in 1718 after a brief, violent struggle.

Agaja led the most important expansions of the kingdom in the 1720s with the conquest of the Kingdom of Allada in 1724 and the Kingdom of Whydah in 1727. Allada and Whydah, both Aja kingdoms, had become important coastal trading centers in the early 1700s, with trade connections to multiple European countries. The two powers made a 1705 agreement where both agreed not to interfere in the trade of the other kingdom. The King of Whydah, Huffon, grew increasingly connected through trade with the British Royal African Company while the king of Allada, Soso, made his ports outposts for the Dutch West India Company. In 1712, a British ship attacked a Dutch ship in the harbor at Allada, triggering economic warfare between Allada and Whydah that lasted until 1720. Upon coming to the throne, Agaja and Soso made an agreement to attack Whydah and remove Huffon from power; however, this plan was halted for unknown reasons.

In 1724, Soso died and a contest for the throne in Allada followed. On March 30, 1724, Agaja's army entered Allada in support of the defeated candidate, named Hussar. After a three-day battle Agaja's army killed the king and set the palace on fire. Rather than place Hussar on the throne, though, Agaja drove him out of the city after establishing his own power. Agaja then turned his forces against the other Aja kingdoms. In April 1724, Agaja conquered the town of Godomey and in 1726 the King of Gomè transferred his allegiance from the King of Whydah to Agaja.

Agaja planned his attack on Whydah in February 1727. He conspired with his daughter, Na Gueze, who was married to Huffon, to pour water on the gunpowder stores in Whydah. He also sent a letter to all of the European traders in the port of Whydah encouraging them to remain neutral in the conflict, in return for which he would provide favorable trade relations at the conclusion of the war. On February 26, 1727, Agaja attacked Whydah and burned the palace, causing the royal family to flee from the city. During the five-day battle, reports say that five thousand people in Whydah were killed and ten to eleven thousand were captured. In April, he burned all of the European factories in the Whydah capital.

In the three years between 1724 and 1727, Agaja had more than doubled the territory of Dahomey, had secured access to the Atlantic coast, and had made Dahomey a prominent power along the Slave Coast.