In this article we theorize the concept of altermundos literacies as they are enacted by parents in transborder families. We demonstrate how these parents, participants in an ethnographic study with mixed-status families in rural Puebla, México, write speculative narratives for their children's futures in ways that reject mononational norms. Altermundos literacies involve the activation of subaltern knowledges across mononational systems and complex literacy practices across papeles to dream-into-action a future designed by and for their lives. We draw on scholars of Black, Indigenous, Chicana, and Latinx futurity to demonstrate how families' altermundos challenge educational practices across borders and dream new possibilities into being. Our findings show how families' dreams navigate constraints imposed by systems of oppression within both the United States and México to varying degrees of success. Regardless, these families' altermundos literacies are invitations for all of us to co-dream futures beyond borders. Implications for researchers, school-based practitioners, and policymakers are discussed.