A seismic coupler includes two coupling members each having an end receptacle for securing to a reinforcing bar of a column of reinforced concrete; and a flexible assembly comprising a cylindrical core formed of alloy, a plurality of steel first rings put on the core, a plurality of flexible second rings put on the core in an alternating fashion with respect to the first rings, a flexible sleeve put on the first and second rings to have both ends being flush with that of the core, two cup-shaped cap members engaged each other to conceal the core, the first and second rings, and the sleeve, and two sets of a plurality of fasteners driven through the coupling members and the cup-shaped cap members into the first rings.