This paper proposes an implantable antenna using a three-dimensional (3-D) multiple-input multiple-output (MIMO) configuration, which operates at 915 and 2450 MHz bands. Both frequency bands are wider enough to cover the important industrial, scientific, and medical (ISM) bands at 915 and 2450 MHz. Using a meandered geometry, introducing capacitive regions between meandered arms and partially slotted ground keeps the antenna dimensions small. Consequently, it consumes an overall volume of 5×5×4.75 = 118.75 mm3 (0.0001λg3, where λg is the guided wavelength at 915 MHz). All elements of the antenna radiate in diversified directions, enabling radiation pattern diversity, resulting in a lower envelope correlation coefficient (ECC) value. Moreover, it provides good gains at both frequency bands (–28.85 dBi at 915 MHz and –20.68 dBi at 2450 MHz). The MIMO channel parameters, and user safety analysis are performed, indicating satisfactory results.Each element of the proposed antenna has a peak SAR value of 306.19 and 252.36 W/Kg at 915 MHz and 2.45 GHz, respectively. The radiation efficiency of the MIMO antenna system is noted to be 0.15 and 0.19% at 915 and 2450 MHz, respectively. When the implant has Br = 120 Mbps (high-speed connectivity), the proposed antenna can cover an area up to 8 and 5.2 m at 915 and 2450 MHz, respectively. The practical validation of the proposed design is performed considering two software-defined radios (SDRs). To the best of the authors’ knowledge, this is the most compact 3-D implantable MIMO antenna that has been designed to date.