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) .44; p = .005; and ITT ES .37; p = .036), in visual analysis time (PP active/placebo difference −0.19; 95% CI −0.05, −0.33; ES .49; p = .013; and ITT ES .40; p = .01), and for boys in phonologic decoding time (PP −0.22; 95% CI −0.03, −0.41; ES .62; p = .004). Children with ADHD‐RS scores above the median showed treatment benefits in visual analysis time (PP ES .8, p = .009), reading speed per word (PP ES .61, p = .008), and phonologic decoding time per word (PP ES .85, p = .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.