Yes. Foxglove (plants in the genus Digitalis, including the common or purple foxglove, Digitalis purpurea) is poisonous. According to Britannica, all parts of the plant contain cardiac glycosides, compounds that act on the heart, and the plant is considered toxic if eaten.
Those same compounds are why foxglove matters in medicine: common foxglove and Grecian foxglove (D. lanata) are grown commercially as the source of the heart drug digitalis, which is made from the dried leaves. Foxgloves are native to Europe, the Mediterranean region, and the Canary Islands and are popular garden flowers, so keep children and pets from chewing on them. If someone swallows part of a plant in the United States, call Poison Control at 1-800-222-1222.
Source: Encyclopaedia Britannica: Foxglove
Image: Digitalis purpurea, by Robert Flogaus-Faust, CC BY 4.0, via Wikimedia Commons.