Acamapichtli (Classical Nahuatl: Ācamāpichtli [aːkamaːˈpit͡ʃt͡ɬi], meaning "Handful of reeds") was the first Tlatoani, or king, of the Aztecs (or Mexica) of Tenochtitlan, and founder of the Aztec imperial dynasty. Chronicles differ as to the dates of his reign: according to the Codex Chimalpahin, he reigned from 1367 to 1387; according to the Codex Aubin, he reigned from 1376 to 1395; and according to the Codex Chimalpopoca, he reigned from 1350 to 1403.

Acamapichtli was not a native of Tenochtitlan. Blood relationships between rulers were an important aspect of politics in 14th century Mexico, and as relative newcomers, the Mexicans were at a disadvantage. On the death of Tenoch, the elders of the Mexica calpultin decided to elect a tlatoani who could secure the fledgling city's position through ties to powerful groups in the region. They sent a delegation to the leaders of Culhuacan. Although the Culhua had only recently ejected the Mexica from Tizaapan, some intermarriage had taken place between the two peoples during their period of association. Acamapichtli was the product of one such union. His father, Opochtli Iztahuatzin, was a Mexican leader, while his mother Atotoztli I was the daughter of the King Coxcoxtli and sister of King Huehue Acamapichtli. He also had ties to the Acolhua of Coatlinchan. In addition to these concrete ties, the Culhua nobility claimed direct descent from the Toltecs, making their bloodline particularly prestigious.

The Spanish colonial historians Diego Durán, Fernando de Alva Cortés Ixtlilxochitl, and Fernando Alvarado Tezozómoc each mention Acamapichtli's maternal uncle, who occupied the throne of Culhuacan in 1324.

Acamapichtli began his rule as cihuacóatl (governor). At the time of his designation he was 20 years old, living in Texcoco with his mother. After his acceptance of the throne, he was brought to Tenochtitlan and made his entry into the city with great pomp. He married Ilancueitl, daughter of the then ruler of Culhuacán, Acolmiztli.

To integrate these ties with the city of Tenochtitlan, Acamapichtli took a wife from each Tenochtitlan calpulli (in addition to his first wife Ilancuetl).

In the same year, Tenochtitlan's sister city of Tlatelolco also installed an outsider as tlatoani — Cuacuapitzahuac, son of Tezozómoc, tlatoani of the Tepanec city of Azcapotzalco, the other major power in the region.

Despite Acamapichtli's Culhua ancestry, his city rapidly fell into the Tepanec orbit and became a tributary of Azcapotzalco. During his reign Mexica forces fought for Azcapotzalco against various city states, notably Chalco, and were eventually allowed to wage war on their own. Expeditions were sent against Cuauhnahuac (modern Cuernavaca) and Xochimilco.

Tribute was due to the Tecpanec rulers every full moon, a tribute said to be oppressive and capricious. In spite of the hostility of Azcapotzalco, Tenochtitlan progressed. The island on which the city was situated (and the only territory subject to its rule) was enlarged to the east with the addition of dirt and rock. The Mexica were careful, however, to maintain a proper distance from the mainland, for defensive purposes in the event of war.