Despite over a century of archaeological research, most models about Teotihuacan’s water management system remain unverified assumptions. Scholars have traditionally assumed that the Puxtla Springs down-valley from the city permitted raised-field chinampa agriculture akin to Postclassic-period Aztec strategies in the southern lakes of the Basin of Mexico. Rainfall has traditionally not been considered to be of much importance, despite its abundance in the Teotihuacano mural corpus and it being the only source of water to the vast majority of the valley. In this paper, we provide a novel ethnoecological model for the interaction between Ancient Teotihucano society and their agricultural landscape, grounded in how indigenous Mesoamerican societies conceived of their relationship with water and the gods. We first argue that rain was fundamental to Teotihuacano agriculture, ideology, political legitimacy, and urban planning contrary to an assumption that permanently-irrigated maize was the principal agricultural strategy in the valley. Though we remain unable to verify or discount the possibility of spring-fed systems providing some amount of food, we document the existence of tiered, large-scale floodwater catchment systems across the Southern and Central Teotihuacan Valley including the Street of the Dead itself. These would play host to massive flooding rituals that legitimized drastically different governance strategies. Furthermore, through epigraphic, architectural iconographic, ethnohistoric, etymological, remotely-sensed, and archaeoastronomic evidence we document Ancient Teotihuacano mountaintop rain-petitioning rituals at the Santuario de Tlaloc (TE-CL-100) site at summit of the Cerro del Patlachique, like those of the Aztec at Mount Tlaloc and the volcano shrines. Finally, we draw attention to an number of parallels between these practices, and similar South American rites.