The alternating direction implicit finite-difference time-domain (ADI-FDTD) formulation was extended to dispersive media-collisionless unmagnetized plasma for the first time. Electromagnetic interactions of collisionless unmagnetized plasma and electromagnetic wave were calculated by using the method. The update equations for collisionless plasma were proposed by using the principle of ADI technique. The unconditional stability of the ADI-FDTD formulation for plasma was analytically derived. The results of numerical experiment confirm that the plasma ADI-FDTD method is consistent with the conventional FDTD method, and its efficiency is higher.