U-spin Symmetry in Charmless B Decays

We prove a general theorem about equal CP rate differences within pairs of U-spin related charmless B and B_s decays. Six pairs of decays into two pseudoscalar mesons are identified where such relations hold. Ratios of corresponding rate differences and certain ratios of rates measure U-spin breaking. These processes provide useful information on the weak phase $\gamma={\rm Arg} V^*_{ub}$. Applications of U-spin symmetry to other decays are discussed.