St. John's Diocesan Girls' H.S. School (informally known as Diocesan or Dio) is a girls-only day school located in Kolkata (Calcutta), West Bengal, India. It was established in 1876 by British missionary Angelina Margaret Hoare from Kent, England who devoted her life to the advancement of women's education in British India. It is a Christian school run by the Protestant Church of North India and its primary language of instruction is English. The school's patron saint is Saint John the Baptist. Although previously a college, the school was stripped of its college status by the British Government when freedom-fighter Bina Das, a student of the school, attempted to assassinate the Governor of Bengal, Stanley Jackson, in the Convocation Hall of the University of Calcutta on 6 February 1932.

Today, the school is often considered to be one of the most prestigious girls' schools in Bengal, and has a rich history of distinguished alumnae such as Lady Abala Bose, Gayatri Chakravorty Spivak, and Maitreyi Devi. It is situated in the iconic Landsdowne Road (officially Sarat Bose Road) in South Kolkata and also runs a school for underprivileged children in Ripon Street.