Matthew Gregory Lewis (9 July 1775 – 16 May 1818) was an English novelist and dramatist, whose writings are often classified as "Gothic horror". He was frequently referred to as "Monk" Lewis, because of the success of his 1796 Gothic novel The Monk. He also worked as a diplomat, politician and an estate owner in Jamaica.

Lewis was the first-born child of Matthew and Frances Maria Sewell Lewis. His father, Matthew Lewis, was the son of William Lewis and Jane Gregory and was born in England in 1750. He attended Westminster School before proceeding to Christ Church, Oxford, where he received his bachelor's degree in 1769 and his master's in 1772. During his time at Westminster, Lewis's parents separated. Mrs Lewis moved to France in this period; while there, she was in continuous correspondence with Matthew. The correspondence between Matthew and his mother consisted of discussion regarding the poor state of his mother's welfare and estate.

That same year, Lewis was appointed Chief Clerk in the War Office. The following year, he married Frances Maria Sewell, a young woman who was very popular at court. She was the third daughter of the senior judge Sir Thomas Sewell and was one of eight children born in his first marriage to Catherine Heath. Her family, like Lewis's, had connections with Jamaica. As a child, she spent her time in Ottershaw. In December 1775, in addition to his War Office post, Lewis became the Deputy Secretary at War. With one exception, he was the first to hold both positions and receive both salaries contemporaneously. Lewis owned considerable property in Jamaica, within four miles of Savanna-la-Mer, or Savanna-la-Mar, which was hit by a devastating earthquake and hurricane in 1779. His son would later inherit this property.

In addition to Matthew Gregory Lewis, Matthew and Frances had three other children: Maria, Barrington, and Sophia Elizabeth. On 23 July 1781, when Matthew was six and his youngest sister one-and-a-half years old, Frances left her husband, taking the music master, Samuel Harrison, as her lover. During their estrangement, Frances lived under a different name, Langley, in order to hide her location from her husband, although he still learned of her whereabouts. On 3 July 1782, Frances gave birth to a child. That same day, hearing of the birth, her estranged husband returned. Afterwards, he began to arrange a legal separation from his wife. After formally accusing his wife of adultery through the Consistory Court of the Bishop of London on 27 February 1783, he petitioned the House of Lords for permission to bring about a bill of divorce. However, these bills were rarely granted and it was rejected when brought to a vote. Consequently, Matthew and Frances remained married until his death in 1812. Frances, though withdrawing from society and temporarily moving to France, was always supported financially by her husband and then later, her son. She later returned to London and then finished her life at Leatherhead, rejoining society and even becoming a lady-in-waiting to the Princess of Wales. Frances and her son remained quite close, with her taking on the responsibility of helping him with his literary career. She even became a published author, much to her son's dislike.

Lewis had a tumultuous relationship with the mistresses his father took on, always siding with his exiled mother in family disputes. His rudeness to such women brought him to the perpetual verge of disinheritance, a threat commonly held over his head and sometimes acted on throughout his lifetime.

Lewis's sisters (especially Lady Lushington) often suggested edits for Lewis's work aimed at making them more acceptable to the public, and went so far as to compose their own drafts of his plays. He rejected all such suggestions.

Lewis was also in a long term relationship with William Kelly, brother of Attorney General for England and Wales Fitzroy Kelly.

Lewis began his education at a preparatory school called Marylebone Seminary under the Rev. Dr John Fountayne, Dean of York. Fountayne was a friend of both the Lewis and Sewell families. There Lewis learned Latin, Greek, French, writing, arithmetic, drawing, dancing, and fencing. He and his classmates were permitted to converse only in French throughout the day. Like many of his classmates, Lewis used the Marylebone Seminary as a stepping stone, proceeding from there to Westminster School, like his father, at the age of eight. There he acted in the Town Boys' Play as Falconbridge in King John and then My Lord Duke in James Townley's farce, High Life Below Stairs.