A pressure sensitive adhesive includes an acryl-based base polymer. The acryl-based base polymer contains an acryl-based segment and a urethane-based segment. In the acryl-based base polymer, the content of the urethane-based segment is 3 to 20 parts by weight based on 100 parts by weight of the acryl-based segment. A pressure sensitive adhesive sheet can be obtained by applying a pressure sensitive adhesive composition on a substrate in a layer form. The pressure sensitive adhesive composition may be curable and may include a polymerization initiator. In formation of the pressure sensitive adhesive sheet, the curable pressure sensitive adhesive composition may be, as necessary.