School-aged children gain weight most rapidly in summer, but few studies have investigated summer weight gain among preschool-aged children. We fit continuous linear spline mixed models to test for accelerated summer weight gain among 2,044 children attending 16 Boston-area Head Start programs between fall 2016 and spring 2019. Academic year and summer rates of change in modified body mass index z-score differed (P < .001), with accelerated summer weight gain most pronounced among children with obesity. As with school-aged children, increased focus on the summer is warranted for promoting healthy weight among children in Head Start.