A grammar school is one of several different types of school in the history of education in the United Kingdom and other English-speaking countries, originally a school teaching Latin, but more recently an academically oriented secondary school, differentiated in recent years from less academic secondary modern schools. The main difference is that a grammar school may select pupils based on academic achievement whereas a secondary modern may not.

The original purpose of medieval grammar schools was the teaching of Latin. Over time the curriculum was broadened, first to include Ancient Greek, and later English and other European languages, natural sciences, mathematics, history, geography, art and other subjects. In the late Victorian era grammar schools were reorganised to provide secondary education throughout England and Wales; Scotland had developed a different system. Grammar schools of these types were also established in British territories overseas, where they have evolved in different ways.

Grammar schools became one of the three tiers of the Tripartite System of state-funded secondary education operating in England and Wales from the mid-1940s to the late 1960s, and continue as such in Northern Ireland. After most Local Education Authorities moved to non-selective comprehensive schools in the 1960s and 1970s, some grammar schools became fully independent schools and charged fees, while most others were abolished or became comprehensive (or sometimes merged with a secondary modern to form a new comprehensive school). In both cases, some of these schools kept "grammar school" in their names. More recently, a number of state grammar schools still retaining their selective intake gained academy status, meaning that they are independent of the Local Education Authority (LEA). Some parts of England retain forms of the Tripartite System, and a few grammar schools survive in otherwise comprehensive areas. Some of the remaining grammar schools can trace their histories to before the 15th century.