Mazzo di Valtellina (German: mud in Valtellina) is a municipality with 1022 inhabitants (as of December 31, 2015) in the Italian province of Sondrio in the Lombardy region and is located in the upper Valtellina. The neighboring municipalities are Grosotto, Monno (BS), Tovo di Sant'Agata and Vervio. The Grisons noble family of the governors of Matsch is probably from Mazzo.