In this Letter, we report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline ferromagnetic Mn5Ge3 as source/drain contacts formed by thermal reactions. Degenerate indium dopants were successfully incorporated into as-grown Ge nanowires as p-type doping to alleviate the conductivity mismatch between Ge and Mn5Ge3. The magnetoresistance (MR) of the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias in the temperature range from T = 2 K up to T = 50 K, which clearly indicated the electrical spin injection from ferromagnetic Mn5Ge3 contacts into Ge nanowires. In addition to the bias effect, the MR amplitude was found to exponentially decay with the Ge nanowire channel length; this fact was explained by the dominated Elliot-Yafet spin-relaxation mechanism. The fitting of MR further revealed a spin diffusion length of lsf = 480 ± 13 nm and a spin lifetime exceeding 244 ps at T = 10 K in p-type Ge nanowires, and they showed a weak temperature dependence between 2 and 50 K. Ge nanowires showed a significant enhancement in the measured spin diffusion length and spin lifetime compared with those reported for bulk p-type Ge. Our study of the spin transport in the Mn5Ge3/Ge/Mn5Ge3 nanowire transistor points to a possible realization of spin-based transistors; it may also open up new opportunities to create novel Ge nanowire-based spintronic devices. Furthermore, the simple fabrication process promises a compatible integration into standard Si technology in the future.