John Joseph Merlin (born Jean-Joseph Merlin, 6 September 1735 – 8 May 1803) was a Freemason, clock-maker, musical-instrument maker, and inventor from the Prince-Bishopric of Liège in the Holy Roman Empire. He moved to England in 1760. By 1766, he was working with James Cox and creating automatons such as Cox's timepiece and the Silver Swan. By 1773, he was designing and making innovative keyboard instruments. In 1783, he opened Merlin's Mechanical Museum in Princes Street, Hanover Square, London, a meeting-place for the gentry and nobility. In addition to his clocks, musical instruments and automata, Merlin is credited with the invention of inline skates in the 1760s. He was referred to by contemporaries as "The Ingenious Mechanic". He was friendly with composer Joseph Haydn.

Jean-Joseph Merlin was born on 6 September 1735, in Huy, in what was then the Prince-Bishopric of Liège and is now in Belgium, Wallonia. His parents were blacksmith Maximilien Joseph Merlin and his wife Marie-Anne Levasseur. He was baptised the same day as he was born, at the parish church of Saint-Pierre-Outre-Meuse in Huy. A broadsheet obituary and later sources give his birthdate incorrectly as 17 September 1735.

Merlin's parents had married in 1732. Merlin was the third of six children; his mother died when he was eight. Merlin's father remarried at least once, to Marie Therese Dechesalle in 1743, and had another child, Charles Merlin. The family moved several times. From ages 19 to 25, Merlin lived in Paris, where he was involved in the Paris Academy of Sciences.

Merlin arrived in England on 24 May 1760, as a technical advisor to the new Spanish Ambassador to London, Joaquín Atanasio Pignatelli de Aragón y Moncayo, conde de Fuentes (15th). As of 1763, Jérôme Lalande recorded that Merlin had helped to complete a large barrel organ which was built for the Princess of Wales. In 1764, he met the 8 year old Wolfgang Amadeus Mozart who tried out the new organ.

By 1766, Merlin was working as a mechanician with British jeweller and goldsmith James Cox. As Cox's chief mechanician, Merlin worked with him to create pieces such as Cox's barometric clock (before 1768) and the Silver Swan (1773). In addition, Merlin acted as a manager and curator of Cox's Jewelry Museum in Spring Gardens, which became a favoured gathering-place of fashionable London between 1772 and 1775.

Horace Walpole referred to the sort of creations displayed by Cox and Merlin as "scientific toys". Fanny Burney's characters visit Cox's museum and debate the significance of such creations in her novel Evelina. Samuel Johnson asserts the underlying importance of such efforts, writing of a visit to Cox's Museum in 1772:

"It may sometimes happen that the greatest efforts of ingenuity have been exerted in trifles; yet the same principles and expedients may be applied to more valuable purposes, and the movements, which put into action machines of no use but to raise the wonder of ignorance, may be employed to drain fens, or manufacture metals, to assist the architect, or preserve the sailor."

By 1773, Merlin was also actively designing and making keyboard instruments. He was granted patent (No. 1081) on 12 September 1774, for a pianoforte stop that could be fitted to a harpsichord. Between 1773 and early 1782, instruments were made to his designs at a workshop at 7 Gresse Street, supervised by Louis Lavigne Verel. One of the combined harpsichord-pianofortes that Merlin manufactured may have been owned by Empress Catherine the Great. A harpsichord-piano from 1779 is in the collection of the Museum of Fine Arts, Boston, and one from 1780 is in Munich's Deutsches Museum. Merlin also experimented with violins and violas.