Sympathetic strings are strings on musical instruments which begin resonating, not due to any external influence such as picking or bowing, but due to another note (or frequency). The effect is most often heard when the fundamental frequency of the string is in unison or an octave lower or higher than the catalyst note, although it can occur for other intervals such as a fifth with less effect.

Musical instruments make use of sympathetic strings to enhance their sound. Some have resonating strings added which exist solely to provide the effect and are not played at all. Common examples of this would be the sitar and the harp, and less common would be specially-fitted guitars. Other instruments such as the piano do not have additional strings, but make use of the effect by allowing their regular strings to vibrate sympathetically when they are not being played directly, particularly when the Damper pedal is used.

Other instruments that have sympathetic strings include:

• gottuvadyam
• some versions of the nyckelharpa
• viola d'amore
• hardingfele
• pianos that use aliquot stringing
• Sitar
• Tambura