Here, we synthesized dextrans modified with trivalent cationic or anionic groups. Aqueous solutions of the cationic and anionic dextrans were then mixed resulting in the formation of polyion complex nanogels (PIC-NGs), which have physically crosslinked salt bridges formed between the cationic and anionic groups. To prepare PIC-NGs in high yield, the content of ionic groups in the cationic and anionic dextrans should be low to avoid interparticle salt bridge formation. The structure of the PIC-NGs is easily affected by the ionic strength of the solution because of shielding of the charges in the ionic groups. However, the conversion of a small amount of the physical crosslinks to covalent crosslinks significantly improved the stability of the PIC-NGs. The covalent crosslinking also stabilized the PIC-NGs against pH change, which will destabilize the salt bridges. These results indicated that the conversion of the small amount of physical crosslinks to covalent crosslinks stabilized the other salt bridges.