Colombo International School (CIS) is a private co-educational school in Colombo, Sri Lanka. It was founded in 1982 by Elizabeth Moir and employs an English-medium curriculum.

In 1998 the school was expanded by the new Chairman of the Board of Directors, Armyne Wirasinha, who created a new branch, Colombo International School Kandy, in Mawilmada.

The school provides a British curriculum-based education system which includes the IGCSE Examination for grades 10 and 11 and International Advanced Level for 6th form.

Colombo International School was established in 1982, shortly after Elizabeth Moir arrived in Sri Lanka.

The school did not initially have a campus but instead operated out of various repurposed residential buildings and homes. Students were trained to sit for the London Ordinary and Advanced Level examinations, before later transitioning to the Pearson Edexcel qualification.

The school is officially registered as a company under the Companies Act of 1982, making its full registered name Colombo International School (Private) Ltd. As an international school, it is under the authority of the Board of Investment of Sri Lanka and is not subject to many of the regulations imposed by the Ministry of Education.

In 1993, the new chairman, Armyne Weerasinghe, initiated a development and expansion programme. In 1998, a new branch, CIS Kandy, was opened.

The school has since expanded to reach the present enrollment of approximately 1600 students from over 30 countries.

The school is split into two branches. The main branch, situated on Gregory's Road, Colombo 7, admits approximately 1600 students per year, with an average of thirty students per class. The Kandy branch is located off Aluthgantota Road, Mawilmada, and admits approximately half that of the main branch: 700 students per year.

The main branch campus lies next to the Sri Sambodhi Maha Viharaya and is split in two by the Ven. Panadure Ariyadhamma Himi Mawatha, which connects the east-facing Gregory's Road to the south-east bound Wijerama Mawatha.

The main campus is split into three sections:

• Infant section

A three-storey building that houses the nursery, playgroup, pre-reception and reception classrooms, as well as those for the first and second grades. The third storey contains a dance room and small supplementary classrooms and three computer laboratories reserved for use by the senior school, this is also where the IT department resides.

• Junior section

A five-storey building with classrooms for the third through sixth grades. It also contains its own separate computer laboratory suitable for twenty students, an art room filled with tables covered in paint and other art provided by the young minds at the junior section and a junior library.

• Senior section

A collection of buildings containing general-purpose classrooms as well as specialised labs for the sciences, namely the Physics lab, ISL Lab, BIO Lab, an art room, along with the head of the section's office. The main senior building overlooks a cement made playground that is used by the school for various events. With access to the MCA grounds that provide the school with a ground to host inter-house competitions basketball practice and cricket.

• Auditorium complex

An appendage of the school's main campus, separated from it by the Ven. Panadure Ariyadhamma Himi Mawatha, containing one main swimming pool and a smaller indoor pool, a few examination halls that when reconfigured can become supplementary classrooms and an auditorium for assemblies, staged productions and other functions.

The Kandy branch is situated next to the Paranagantota Road.

The campus houses facilities for sports such as netball, basketball and cricket, as well as those for the performing arts and yearly staged productions.

In 2010, the National Child Protection Authority commenced a probe against the school over the use of a sexual education textbook, Introducing Moral Issues by Joe Jenkins, in the school's sixth and fifth-grade curriculum. The probe was a result of former UNP Member of Parliament and lawyer Dinesh Dodangoda lodging a complaint with the NCPA after failing to persuade the school to remove the book from its curriculum. NCPA Chairwoman Anoma Dissanayake urged the Ministry of Education to take legal action against the school over the book's depictions of contraceptives, abortion and sexual intercourse. The school continued to defend the book and refused to remove it on the grounds that it "educated children on sex issues so that they will be better prepared for the future".

In September 2018, Saakya Rajawasan, a student at CIS, was barred from participating in the senior fashion show due to attending a rehearsal wearing a rainbow flag cape, after previously having been told that the flag would not be tolerated. She was again subject to disciplinary action after attending school the following day with the cape draped over her school bag, with her behaviour being deemed 'unacceptable and irresponsible' by the acting principal.

The events that transpired were reported on with special interest by the Colombo Telegraph, an online news publication that went on to release a series of articles in support of Rajawasan. On October 8, Rajawasan issued a public statement in which she further alleged that she was denied an opportunity for prefectship on the grounds that she chose to wear trousers instead of a skirt, which went against the school's uniform for girls.

In 2020, Shakya Ranatunga became the first student in Sri Lanka to have manufactured a drone and successfully obtained registration from the Civil Aviation Authority of Sri Lanka. The lithium-ion powered drone had a maximum flight time of 10 minutes.

On June 2 2024, two 15-year-old students of CIS fell to their deaths from the 67th floor of the Altair Apartments building in what authorities suspected to be a double suicide.

• Dr.Yanushi Dullewe Wijeyeratne, Cardiology Specialty Registrar with PhD at the St George's University Hospitals NHS Foundation Trust, London, UK & winner of the 2012 Bedi Prize.
• Angajan Ramanathan, Deputy Chairman of Committees of Parliament and Member of Parliament.
• Anarkalli Aakarsha Jayatilaka, film and teledrama actress, model, member of Southern Provincial Council (2009–present).
• Gajendrakumar Ponnambalam, Member of Parliament - Jaffna (2001-2010)
• Sujit Sivasundaram, Professor of World History, University of Cambridge
• Asha de Vos, marine biologist, ocean educator and pioneer of blue whale research within the northern Indian Ocean, known for Blue Whale Project; Senior TED Fellow and chosen for a BBC 100 Women award in 2018.
• Anuk Arudpragasam novelist writing in English and Tamil. Winner of the DSC Prize for South Asian Literature, and shortlisted for the Booker Prize

• Colombo International School Official Website