A calibration source employing 8Li ( t1/2=0.838 s) has been developed for use with the Sudbury Neutrino Observatory (SNO). This source creates a spectrum of β-particles with an energy range similar to that of the SNO 8B solar neutrino signal. The source is used to test the SNO detector's energy response, position reconstruction and data reduction algorithms. The 8Li isotope is created using a deuterium-tritium neutron generator in conjunction with a 11B target, and is carried to a decay chamber using a gas/aerosol transport system. The decay chamber detects prompt α-particles by gas scintillation in coincidence with the β-particles which exit through a thin stainless steel wall. A description is given of the production, transport, and tagging techniques along with a discussion of the performance and application of the source.