A new flexible ultrawideband (UWB) antenna is presented for wearable applications in the 3.7-10.3 GHz band, which is highly tolerant to human body loading and physical deformation. The antenna exhibits a footprint of 80 mm × 67 mm and is based on a simple microstrip structure with two modified arc-shaped patches as the main radiator. A full ground plane is maintained on the opposite side of the substrate to suppress antenna loading from the underlying biological tissues and back radiation directed toward the human body. For enhanced flexibility and robustness, the proposed antenna is realized using conductive fabric embedded into polydimethylsiloxane polymer. Promising simulation and experimental results are presented for free-space and in-vitro wearable scenarios. To our knowledge, this is the first UWB antenna with a full ground plane that is concurrently highly tolerant to harsh operating conditions, such as those encountered in wearable applications.