The main objective of this letter is to present a small-size, dual-wideband implantable antenna operating in the Medical Device Radiocommunications Service (MedRadio) (401-406 MHz) and Industrial, Scientific, and Medical (ISM) (2.4-2.48 GHz) bands. The proposed antenna has a 71.6% reduction in size with respect to the previous similar dual-band implantable antenna. The measured -10-dB bandwidths are 56% (278 MHz) for the MedRadio and 33% (870 MHz) for the ISM bands, respectively. The antenna is in vitro tested in a tissue-mimicking gel approximating the electrical properties of human skin tissue. Results are compared to simulations.