We present results from the first randomized experiment of a remedial inquiry-based science education program for low-performing elementary students in a developing country. Among third-grade students in 48 low-income public elementary schools in Metropolitan Lima who score in the bottom 50% of their school baseline science distribution, half are randomly assigned to receive remedial inquiry-based science education in after-school sessions, and the remaining half to business as usual control conditions. Assignment to treatment increased endline science achievement by 3 percentiles (0.12 SD) with greater gains for students who attended at least one remedial session, and a concentration of gains among boys. We cannot reject the null hypothesis of no indirect science achievement gains among nonparticipants.