Background: Previous research has shown positive effects of Omega 3/6 fatty acids in children with inattention and reading difficulties. We aimed to investigate if Omega 3/6 improved reading ability in mainstream schoolchildren. Methods: We performed a 3-month parallel, randomized, double-blind, placebo-controlled trial followed by 3-month active treatment for all subjects. Mainstream schoolchildren aged 9-10 years were randomized 1:1 to receive three Omega 3/6 capsules twice daily or identical placebo. Assessments were made at baseline, 3 months, and 6 months. The primary outcome measure was the Logos test battery for evaluating reading abilities. The trial is registered with ClinicalTrials.gov, number NCT02557477. Results: The study enrolled 154 children (active n = 78; placebo n = 76), of whom 122 completed the first 3 months (active n = 64; placebo n = 58) and 105 completed the whole study (active/active n = 55; placebo/active n = 50). Outcomes were assessed by per protocol (PP) and intention-to-treat (ITT) analyses. Active treatment was superior to placebo at 3 months for improvement in phonologic decoding time (PP active/placebo difference -0.16; 95% CI -0.03, -0.29; effect size (ES) 0.44; p = 0.005; and ITT ES 0.37; p = 0.036), in visual analysis time (PP active/placebo difference -0.19; 95% CI -0.05, -0.33; ES 0.49; p =0.013; and ITT ES 0.40; p = 0.01), and for boys in phonologic decoding time (PP -0.22; 95% CI -0.03, -0.41; ES 0.62; p = 0.004). Children with ADHD-RS scores above the median showed treatment benefits in visual analysis time (PP ES 0.8, p = 0.009), reading speed per word (PP ES 0.61, p = 0.008), and phonologic decoding time per word (PP ES 0.85, p = 0.006). Adverse events were rare and mild, mainly stomach pain/diarrhea (active n = 9, placebo n = 2). Conclusions: Compared with placebo, 3 months of Omega 3/6 treatment improved reading ability--specifically the clinically relevant "phonologic decoding time" and "visual analysis time"--in mainstream schoolchildren. In particular, children with attention problems showed treatment benefits.