The King's School is an 11–18 boys grammar school with academy status, in the market town of Grantham, Lincolnshire, England. The school's history can be traced to 1329, and was re-endowed by Richard Foxe in 1528. Located on Brook Street, the school's site has expanded over the course of its history, with some school buildings dating back to 1497.

Today, King's remains an all-boys grammar school, with just over 1,000 pupils. King's and another Grantham grammar school, Kesteven and Grantham Girls' School, share teaching resources for sixth form study in certain subjects; since 2006–2007, timetables for this study have been co-ordinated between both schools.

The King's School has an unbroken history on the same site since its re-endowment in 1528 by Richard Foxe, although its history can be traced back to 1329. Sir Nikolaus Pevsner in his Buildings of England, dates the original School building to 1497. Foxe was born and raised locally. He entered into the service of Henry Tudor, while Henry was in exile in Brittany. When Henry took the throne of England as Henry VII, Foxe became his secretary, and later founded Corpus Christi College, Oxford (1517) and Taunton Grammar School (1522). In the 16th century the School became known as the Free Grammar School of King Edward VI. Scholars numbered perhaps a few dozen at foundation, and there were still fewer than one hundred until the 20th century.

Isaac Newton was a King's School scholar between 1655 and 1660. As was customary in his time, he carved his signature on the wall of what is now the school library, although the signature has never been confirmed as authentic; visitors from around the world come to view this indication of Newton's education. A replica of the signature is on display in Grantham Museum.

The school became a selective state grammar school under the implementation of the Education Act 1944. By 1970, Kesteven County Council announced plans to turn its grammar schools into co-educational comprehensives for ages of 11–16 and leave Grantham College as the town's only sixth form. Other parts of Kesteven became comprehensive but responsibility for education passed to Lincolnshire County Council under the Local Government Act 1972, and King's remained a grammar school.

The novelist and eccentric Frederick Rolfe ('Baron Corvo') was briefly a teacher at the school.

In 1988 an illustrated history of the School was published by a former King's School teacher: The King's School Grantham – 660 years of a Grammar School.

On 1 August 2011 The King's School ended its long relationship with the local elected authorities and the town of Grantham, by converting to a selective academy. It remains a selective boys' school and has kept its name and logo.

The school provides subjects that are traditional in range, providing curriculum subjects with breadth, depth and challenge at each Key Stage throughout the school. Boys attending the school are provided with opportunities to partake in extra-curriculum and academic experiences such as in sports, music, art, drama, chess, the Duke of Edinburgh award scheme and languages. Additionally, the school offers a range of lunchtime and after school clubs to enhance boys' learning experiences. Boys may also wish to partake in a programme of visits and trips overseas with the school.

Form tutors support the progress and wellbeing of boys throughout the school year, together with subject staff and Heads of Year staff at the school. Subjects offered by the school include Art, Business, Computing, Drama, English, French, History, Music, Physical Education (PE), Politics, Religious Education (RE), Biology, Chemistry, Design and Technology, Economics, Financial Studies, Geography, Mathematics, Physics, Psychology, Science and PSHE Curriculum.

King's boys generally take ten to eleven General Certificate of Secondary Education (GCSE) examinations in Year Eleven (aged 15–16), and they have a choice of three or four A-levels in the sixth form. An Office for Standards in Education, Children's Services and Skills (Ofsted) inspection in May 2008 graded The King's School as "outstanding". The majority of students go on to higher education following the completion of their A-levels at the end of Year Thirteen (aged 17–18), and it is common for several boys to gain Oxbridge offers.

An agreed school uniform is in place at The King's School, as determined by the schools School Uniform policy. The school expects all boys attending the school to be "clean, neat and tidy in their appearance", with an expectation to be wearing full school uniform during their commute to and from the school, as well as when representing the school at occasions outwith the school building and grounds.

Boys are expected to have their shirts tucked in to their school trousers, with clean and "well groomed" hair, having no "extremes of length, style or colour", the school expects boys to have "natural coloured" hair".

The current agreed school uniform for boys attending The King's School is as follows:

• Black blazer with school badge
• Formal, white school shirt
• Black or dark grey, long school trousers
• School tie
• Dark grey or black socks
• Black school shoes (not suede material, boots and trainers are forbidden)
• Grey school pullover jumper with maroon stripe, or;
• Plain grey v-neck pullover jumper

A selective academy, boys wishing to attend The King's School must have achieved at least the minimum standardised qualifying score for entry in the examination in order to be classed as eligible for a place to attend the school. The school selects boys to admit based on their academic ability and performance, and leaves spaces unfilled if there are not enough boys who have reached the necessary qualifying standard which is a standardised score of 220 or above.

The schools Published Admission Number for boys in Year 7 is currently a maximum of 174 boys per year, with the number for external Sixth Form applications being a maximum of 30 boys.

• William Cecil (statesman) (1530s)
• Henry More (philosopher) (c. 1620s)
• Isaac Newton (scientist) (1655–1659)
• Colley Cibber (playwright and poetaster) (1682–1687)

• Captain Albert Ball VC DSO MC (World War I Flying Ace)
• Air Vice Marshal Gary Waterfall CBE (Senior RAF Officer, Chief of Staff (Operations) Permanent Joint Headquarters)
• Jason Lai (Orchestral Conductor and TV Presenter)
• Andy Bond, Former Asda COO
• Andy Clarke, Former Asda CEO
• Brian Thompson (1938–2011), footballer and concert promoter
• Ben Everitt (1978–), MP for Milton Keynes North.

• List of the oldest schools in the United Kingdom

• The King's School, Grantham
• The King's School Old Boys' Society