An adjustable thermally insulating beverage jacket has two fastener portions disposed at opposite ends of a flexible body portion, which can be attached to each other in order to accommodate a beverage container. The flexible body portion is made of a thermally insulating material to keep beverages cold or warm. A handle insertion portion at one end of the flexible body portion allows use with beverage containers that have handles. A base connected to the bottom edge of the flexible body portion provides a support surface for the bottom of the beverage container.