There are provided an anionic polymer, a polyion complex, and a ternary polymer composite, each of which is stable in a biological environment and is capable of realizing small RNA delivery without causing any undesired immune response. An anionic polymer according to an embodiment of the invention includes: a main chain which includes repeating units having carboxyl groups and a side chain which is linked with part of the carboxyl groups in the main chain and is represented by the following formula:-A-B—Xwhere : A represents a residue having one or more aminoethyl bonds B represents an in vivo cleavable bond and X represents a small RNA. A polyion complex according to an embodiment of the invention includes the anionic polymer as described above and a cationic polymer. A ternary polymer composite according to an embodiment of the invention includes the polyion complex as described above and a charge conversional polymer.
