I offer a phenomenological description of undocumented immigrant reason, provisionally understood as a sort of historical reason grounded on undocumented immigrant life. That is, the categories of undocumented immigrant reason are resources for undocumented immigrant existence and are inscribed in the historical memory of immigration (they are shared and communal), accessed by immigrants in stories, anecdotes, and interpersonal trauma. Abstracting from personal experience, testimony, popular culture, and elsewhere, I propose a fragmentary list of these categories of undocumented immigrant reason, a list that includes journeying, crossing/nepantla, uncertainty/zozobra, nostalgia, and return. These categories, which structure undocumented immigrant reason, are reflected in beliefs and attitudes about migration, belonging, the contingency of life, the centrality of memory, and the meaning of death. Constitutive of immigrant rationality are beliefs and ways of being that lend meaning to immigrant life, including those beliefs and ways of being that place immigrants in closer proximity to fundamental human truths.