Johann Kaspar Mertz (August 17, 1806 - October 14, 1856) was an Austrian-based guitarist and composer, born in Pressburg, now Bratislava, Slovakia.

Mertz was active in Vienna, which had been home to such important guitarists as Anton Diabelli, Mauro Giuliani, Wenceslaus Matiegka and Simon Franz Molitor , among others. Mertz was a guitar virtuoso who established a solid reputation as a performer. It is rumoured that Mertz's death was caused by an accidental overdose of prescription medicine, administered by his wife.

Mertz's guitar music, unlike that of most of his contemporaries, followed the pianistic models of Chopin, Mendelssohn and Schumann, rather than the classical models of Mozart and Haydn (as did Sor and Aguado), or the bel canto style of Rossini (as did Giuliani).

The Bardenklänge (1847) are probably Mertz's most important contribution to the guitar repertoire—a series of deceptively difficult character pieces in the mould of Schumann.