Smooth polystyrene (PS) and polyamide 12 (PA 12) surfaces were produced via an injection molding process followed by a smoothing process and subsequently treated with O2 plasma to increase the number of polar groups capable of hydrogen bond formation on the surface. The presence of related groups was evident from XPS and contact angle measurements. The sample topographies were investigated by AFM. The plasma treatment allowed the joining of the substrates without adhesive by pressing the substrates together below or around Tg. Notably, not only substrates of the same polymer but also polystyrene and polyamide 12, which are incompatible, were joined with this method. The adhesion between the substrates was determined using a LUMifrac apparatus. Adhesive strengths up to 5.5 MPa were obtained. Remarkably, the joint substrates could be rapidly de-bonded on demand simply by treatment with water, and the separated substrates could be re-bonded by renewed plasma treatment. This bonding technique shows the potential of adhesive-free reversible bonding of polymers which could help to increase the recyclability of polymers and therefore reduce polymeric waste.