The synthesis of optical quality β-barium borate microwires and nanowires (MNWs) is reported using an organic-free hydrothermal method with BaCl2·6H2O, NaOH, and H3BO3 as source materials, and assisted with post-annealing. As-synthesized MNWs, with diameters ranging from 500 nm to 2 μm and lengths up to several hundred micrometers, show good optical-waveguiding capabilities. Based on evanescent coupling between a single BBO MNW waveguide and a fiber taper, propagation losses of 0.30 dB μm−1 (at 532 nm) and 0.21 dB μm−1 (at 671 nm) are evaluated, respectively. An evident second-harmonic generation (SHG) signal at 532 nm with a measured conversion efficiency of about 8.4% is observed when excited by waveguided 1064 nm, picosecond laser pulses within a BBO MNW with a length of the order of 100 μm. The dependence of the SHG conversion efficiency on the MNW diameter is also investigated. These results show a much-higher SHG efficiency for BBO single-crystal MNWs compared with bulk crystal, which suggests potential applications in future micro-/nanoscale nonlinear optical applications such as optical modulation and frequency conversion.