A new and distinct cultivar of Chrysanthemum plant named `Yosnowmass`, characterized by its upright and mounded plant habit; vigorous and strong growth habit; large dark green-colored foliage; uniform flowering response and habit; early flowering habit; large decorative incurved-type inflorescences with elongated oblong-shaped and white-colored ray florets; and good postproduction longevity with plants maintaining good substance and color for about three to four weeks in an interior environment.