Describing the theft as a "tragic loss", the US band have pleaded for the instrument's safe return after it was nabbed off the stage during their set at Manchester Academy.
The band said: "This instrument has little monetary worth, but provides a wealth of sentimental value to us.
We are prepared to offer healthy reward upon its return. In addition, if the Mandolin is received before we play Reading or Leeds, the person who delivers it will be offered tickets to those events.
"This is truly a sad day for the Spinto Band. If you have any information regarding its whereabouts or wish to express your sympathy, please contact us."
The instrument in question is a Kay electric mandolin with a wood sunburst finish, white pickgaurd, and a crack in the headstock.
Anyone with info should contact email@example.com.