Lionel Tertis (December 29, 1876 - February 22, 1975) was a English violist and one of the first viola players to find international fame.

Tertis was born in Hartlepool and initially studied the violin in Leipzig and at the Royal Academy of Music in London. There he was taught by Alexander Mackenzie who encouraged him to take up the viola instead. Under the additional influence of Oskar Nedbal , he did so and rapidly became one of the best known violists of his time, touring Europe and the USA as a soloist. Composers such as Arnold Bax and Frank Bridge wrote pieces specially for him.

Tertis played a particularly large viola in an attempt to get an especially rich tone from his instrument. Troubled by the awkwardness of an 18 inch viola, he created his own Tertis model, which provides many of the tonal advantages of the larger instrument in a manageable 16-3/4 inch size. He also arranged several pieces not originally for the viola for his instrument, such as Edward Elgar's Cello Concerto.

Tertis was the author of a number of publications about string playing, the viola in particular, and his own life. They include Cinderella No More and My Viola and I.

Lionel Tertis died in Wimbledon, London.