Background: Eye health knowledge drives appropriate behaviours that are favourable to healthy vision. Evaluating the eye health knowledge of school children aids in identifying information gaps and improving knowledge on eye health education. Objectives: To determine the level of eye health knowledge and also the effectiveness of an eye health education intervention among secondary school students in Onitsha, Nigeria. Methodology: A before-and-after study design was used to assess the efficacy of eye health education among secondary school students. A questionnaire was used to assess eye health knowledge among students. The same questionnaire was re-administered after an educational intervention. The comparisons between means was done using T-test while Chi-square test was used to analyse association between categorical variables. Results: A total of 296 students made up of 178 (60.1%) males and 118 (39.9%) females with a mean age of 14.7 ± 1.3 years (Range 12-19 years), took part in this study. There was a fair knowledge score of 13.67 ± 3.75 (54.7%) before the educational intervention. The eye health knowledge post-intervention score improved to 19.33 ± 3.38 (77.3%), representing a good knowledge (p = 0.05). Students in junior secondary school had a statistically significant higher mean change in knowledge score than those in senior secondary school (t = 2.46; p = 0.015). There was no statistically significant difference in mean change in knowledge score between males and females (t = 0.370, p =0.599). Conclusion: Eye health education increased the awareness and knowledge of healthy eye behaviours and good eye practices among the secondary school students. We recommend the use of posters, audio-visual displays, role plays and demonstrations in addition to lectures to enhance eye health knowledge among secondary school students.