Louis-Joseph Diémer (14 February 1843 – 21 December 1919) was a French pianist and composer. He was the founder of the Société des Instruments Anciens in the 1890s, and gave recitals on the harpsichord. His output as a composer was extensive, including a piano concerto and a quantity of salon pieces, all more or less forgotten these days.
Diémer was born and died in Paris. He studied at the Paris Conservatoire, winning premiers prix in piano, harmony and accompaniment, counterpoint and fugue, and solfège, and a second prix in organ. His teachers were Antoine Marmontel for piano, Ambroise Thomas for composition and François Benoist for organ.
He quickly built a reputation as a virtuoso and toured with the violinist Pablo de Sarasate. At the Conservatoire he taught, among others, Édouard Risler, Alfred Cortot, Lazare Lévy, Alfredo Casella, Yves Nat, Marcel Ciampi, José Cubiles and Robert Casadesus.
In 1888, Diémer succeeded Marmontel as professor of piano at the Paris Conservatory. He was also instrumental in promoting the use of historical instruments, giving a series of harpsichord performances as part of the 1889 Universal Exhibition and contributing to the founding of the Société des Instruments Anciens.
César Franck's Symphonic Variations, Jules Massenet's only piano concerto, Camille Saint-Saëns's 5th Piano Concerto, and Édouard Lalo's Piano Concerto in F minor were all dedicated to Diémer. Pyotr Ilyich Tchaikovsky's 3rd Piano Concerto was intended to be dedicated to him.