We examine the effect of the expansion of charter schools on socioeconomic segregation in American public education. Using a district-level panel data set from 1998 to 2015, we describe and model changes in within-district segregation of low-income students, proxied by free-lunch eligibility (FLE). We show that the segregation of FLE students from non-FLE students increased by about 15% in large school districts and find that charter school penetration and growth played a role in increasing socioeconomic segregation within districts. We estimate that a one standard deviation increase in charter enrollment rates increases the dissimilarity index in a district by 6% of a standard deviation. Although this impact is modest, we do find that segregation is more pervasive in the charter sector. So, continued growth of charter schools could exacerbate socioeconomic segregation.