Sigma Guitars is a guitar manufacturing brand originally released by C.F. Martin & Company as a line of guitars at affordable prices to compete with the increasing number of imported guitars from Japan and elsewhere. The Sigma line was discontinued by Martin in 2007. The rights to the name were acquired by German company AMI Musical Instruments GmbH, which relaunched the brand with guitars being produced in China.

Since the U.S. rights to the Sigma brand name and trademark had been allowed to lapse by C.F. Martin & Co. prior to selling the brand to AMI, and had since been picked up by St. Louis Music of Missouri, AMI is not allowed to use the brand name "Sigma" in the U.S. for their current line of guitars. They were distributed and sold in the U.S. as Kindred Guitars.

The current range of Sigma instruments include acoustic and classical guitars, acoustic basses and ukuleles.