The resistivity and Hall coefficient of single-crystal UNi₂Si₂ have been studied in detail for the temperature range 4.2-300 K. The resistivity of UNi₂Si₂ is largely due to magnetic scattering and the phonon scattering contribution is estimated to be about 14% at room temperature. At low temperatures, the resistivity can be described by a gapped spin-wave model plus a T² term. The temperature dependence of the Hall coefficient is accounted for by a theoretical model invoking skew scattering of conduction electrons by localized magnetic moments. Among the three magnetic phase transition temperatures, the two lower ones are found to be magnetic field dependent and shift with the field applied along the tetragonal c axis. Using the resistivity measurement in an applied magnetic field, a field-temperature phase diagram of UNi₂Si₂ is presented.