A piston assembly for a shock absorber includes: a bypass passage formed along a vertical length direction of a piston rod in a small-diameter portion that is stepped in a lower portion of the piston rod reciprocating within a cylinder; a main piston connected to the small-diameter portion; an orifice assembly coupled to the small-diameter portion and disposed under the main piston to form an orifice passage, which communicates with the bypass passage; a housing coupled to the small-diameter portion and disposed under the orifice assembly, the housing having an opened upper side and forming a pressure chamber; and a sliding valve coupled to the orifice assembly and accommodated in the pressure chamber to generate a damping force by selectively opening and closing the orifice passage while reciprocating through a high-frequency section and a low-frequency section. Accordingly, the piston assembly is capable of improving a ride comfort by exhibiting a damping performance over a low-frequency section and a high-frequency section.