There is little knowledge about the effects of kindergarten Spanish phonological awareness interventions on literacy outcomes of grade one children from vulnerable environments. To fill this gap, this study evaluates the effects of a kindergarten phonological awareness intervention on grade one literacy skills in Spanish speaking children (N = 178) from low-income neighborhoods in Argentina. The three-month program delivered at the beginning of kindergarten included 34 lessons focused on phoneme segmentation, categorization, and blending, and letter-sound knowledge. Using a pretest-posttest comparison-group design, measures of phonological awareness, word reading, and spelling were collected before the intervention, immediately after the intervention, and one year later when children were finishing grade one. In addition, reading comprehension was examined at the end of grade one. Results reveal that the intervention had a significant effect on phonological awareness skills, letter-sound knowledge, word reading, spelling, and reading comprehension. These findings suggest that a systematic, phonological awareness intervention program delivered during kindergarten to native speakers of Spanish has a positive impact on their grade one literacy outcomes.