Amino semitelechelic poly(N-isopropylacrylamide) (PNIPAAm) was prepared by radical polymerization with aminoethanethiol hydrochloride as a chain-transfer agent. Semi-interpenetrating polymer network (semi-IPN) hydrogels, composed of alginate and amine-terminated PNIPAAm, were prepared by crosslinking with calcium chloride. From the swelling behaviors of semi-IPNs at various pH's and Fourier transform infrared spectra at high temperatures, the formation of a polyelectrolyte complex was confirmed from the reaction between carboxyl groups in alginate and amino groups in modified PNIPAAm. Semi-IPN hydrogels reached an equilibrium swelling state within 24 h. The water state in hydrogels, investigated by differential scanning calorimetry, showed that sample CAN55 [alginate/PNIPAAm (w/w) = 50/50] exhibited the lowest equilibrium water content and free water content among the hydrogels tested, which was attributed to its more compact structure compared to other samples and the high content of interchain bonding within the hydrogels. Alginate/PNIPAAm semi-IPN hydrogels exhibited a reasonable sensitivity to the temperature, pH, and ionic strength of swelling medium.