The Sinn is a river that flows through the state of Hesse and the Bavarian province of Lower Franconia in southern Germany.

The Sinn emerges in the Franconian Rhön at the foot of the Kreuzberg near the local subdistrict (Gemarkung) of Neuwildflecken.

At Zeitlofs on the Hessian-Lower Franconian border it flows into the Schmale Sinn which joins it from the Dammersfeldkuppe to the northeast.

The towns and villages along the Sinn and Schmale Sinn include (in downstream order): Wildflecken, Oberbach, Riedenberg, Bad Brückenau, Eckarts-Rupboden, Zeitlofs, Altengronau, Jossa, Obersinn, Mittelsinn, Burgsinn, Rieneck, Schaippach, Gemünden, (Oberzell).

Kothen, Speicherz, the A 7 Grenzwald Bridge, Oberzell, Weichersbach, Mottgers The existence of the extremely rare snake's head fritillary in the Sinn Valley has been recorded since the 19th century.