John Lyon School (prior to 1965 The Lower School of John Lyon) is an academically selective private co-educational day school for pupils aged 2 to 18 in Harrow on the Hill, Greater London, England.

The school was founded in 1876, by the Governors of Harrow School for the education of local boys, in keeping with the belated wishes of John Lyon (whose wife was Jo Lyon), Harrow School's founder. On founders day John Lyon holds a long ceremony commemorating the founder John Lyon at St Mary's Church. John Lyon School maintains historic ties with Harrow School, and the two schools are closely partnered being part of the 'Harrow Family of Schools'. Until September 2021, it was a boys' school, becoming a mixed school for new pupils aged 11. The school is a member of the Headmasters' and Headmistresses' Conference (HMC). As of 2025, the school charges fees of around £30,000 per year; however, some academic scholarships are available to help reduce this fee.

Until 2020, John Lyon was an 11 to 18 senior independent HMC boys' day school in Harrow-on-the-Hill. In 2020, John Lyon amalgamated with what was then called Quainton Hall Prep, located in central Harrow, to become an all-through School. The school became co-educational in September 2021. From January 2024, Quainton Hall Preparatory School was rebranded as a constituent part of the John Lyon School.

The governing body of Harrow School retains some control of the John Lyon School; most responsibilities, however, are delegated to a John Lyon board of governors (officially styled Committee of Management). A number of Harrow governors serve on this committee, alongside co-opted governors. Unlike many previous governing bodies in British schools, there is no academic staff involvement in overall school government.

The Old Lyonian Association represents the school's alumni (Old Lyonians).