We report the observation of B-→J/ψΛp̄ and searches for B-→J/ψΣ0p̄ and B0→J/ψpp̄ decays, using a sample of 275×106 BB̄ pairs collected with the Belle detector at the Υ(4S) resonance. We observe a signal of 17.2±4.1 events with a significance of 11.1σ and obtain a branching fraction of B(B-→J/ψΛp̄)=11.6±2.8(stat)-2.3+1.8(sys)×10-6. No signal is found for either of the two decay modes, B-→J/ ψΣ0p̄ and B0→J/ψpp̄, and upper limits for the branching fractions are determined to be B(B-→J/ψΣ0p̄)<1. 1×10-5 and B(B0→J/ψpp̄)<8.3×10-7 at 90% confidence level. © 2005 The American Physical Society.