The purpose of this invention is to obtain a water-based ink composition for a ballpoint pen excellent in writing capabilities expressing continuous and good ink flow without being scratchy and writing impossibility when provided by filling in a ballpoint pen having various ballpoint chips and provide a water-based ballpoint pen. The water-based ink composition for a ballpoint pen comprises a colorant, water, a water-soluble organic solvent and a polysaccharide as a shear thinning agent. In the composition, the polysaccharide exists in a particle state having <5[mu]m average particle size in the water-based ink composition for a ballpoint pen.