The performance measures from the Progress in International Reading Literacy Study revealed no measurable growth in the U.S. average reading scores of 4th graders for the period between 2011 and 2016. Therefore, the diverse learning needs of students need to be customized if the education system is to accomplish its goal to graduate well-informed individuals capable of sustaining a lifestyle conducive to a democratic society. The purpose of this study was to investigate the effect of differentiated instruction on 3rd graders' reading comprehension. The theoretical foundation for this teaching approach was Vygotsky's zone of proximal development. The research question focused on a comparison of differentiated and nondifferentiated instruction and the effects on 3rd graders' Partnership for Assessment of Readiness for College and Careers (PARCC) reading comprehension scores. In this cross-sectional, quasi-experimental causal comparative study with N = 128 3rd-grade students, archival PARCC reading scores for the years 2015, 2016, and 2017 were analyzed with several 2-tailed independent sample t tests to determine the differences between the groups. The students in the experimental group (n = 64) received differentiated instruction, students in the non-experimental group received nondifferentiated instruction (n = 64). The duration of the intervention lasted for the entire school year. The statistical analysis results revealed that differentiated instruction significantly improved the students' PARCC reading scores in all 3 years. The intimation for positive social change is allowing stakeholders to provide opportunities to teachers to learn and apply differentiated instruction for their students, thus helping them to become proficient readers, which in turn might increase the human capital contributing to and competing in a global society.