We report the first observation of Ξ⁻ in heavy ion collisions at the AGS. The lifetime of Ξ⁻ measured agrees very well with the particle data group value. We present the observed Ξ⁻ effective mass spectrum and the rapidity distribution after acceptance correction. In the measured rapidity region (1.4−2.9) for 14.6 GeV/c Si on Pb, we obtained a yield of 0.25 ± 0.04 Ξ⁻ per central event and a ratio of N(Ξ⁻)/N(Λ) = 0.12 ± 0.02 using a fireball⁺ model (a model which includes a fireball plus a term simulating a cascade) for acceptance correction (see text for systematic error). This result is larger than any cascade model prediction so far.