A new and distinct cultivar of Chrysanthemum plant named `Yochatham`, characterized by its compact, upright, outwardly spreading and mounded plant habit; strong and freely branching growth habit; dark green-colored foliage; uniform flowering response and habit; early and freely flowering habit; decorative-type inflorescences with elongated oblong-shaped and light purple-colored ray florets; and good postproduction longevity with plants maintaining good substance and color for about there weeks in an interior environment.