The rate constant for Cl + NH3 -> HCl + NH2 has been measured over 290-570 K by the time-resolved resonance fluorescence technique. Ground-state Cl atoms were generated by 193 nm excimer laser photolysis of CCl4 and reacted under pseudo-first-order conditions with excess NH3. The forward rate constant was fit by the expression k(1) = (1.08 +/- 0.05) x 10(-11) exp(-11.47 +/- 0.16 kJ mol(-1)/RT) cm(3) molecule(-1) s(-1), where the uncertainties in the Arrhenius parameters are +/- 1 sigma and the 95% confidence limits for k(1) are +/- 11%. To rationalize the activation energy, which is 7.4 kJ mol(-1) below the endothermicity in the middle of the 1/T range, the potential energy surface was characterized with MPWB1K/6-31++G(2df, 2p) theory. The products NH2 + HCl form a hydrogen-bonded adduct, separated from Cl + NH3 by a transition state lower in energy than the products. The rate constant for the reverse process k(-1) was derived via modified transition state theory, and the computed k(-1) exhibits a negative activation energy, which in combination with the experimental equilibrium constant yields k(1) in fair accord with experiment.