The conformational transition from right-handed B–DNA to left-handed Z–DNA—the B–Z transition—has received increased attention recently because of its potential roles in biological systems and its applicability to bionanotechnology. Though the B–Z transition of poly(dG–dC) · poly(dG–dC) is inducible under high salt concentration conditions (over 4 M NaCl) or by addition of multivalent cations, such as hexaamminecobalt(III), no cationic polymer were known to induce the transition. In this study, it is shown by circular dichroism and UV spectroscopy that the cationic comb-type copolymer, poly(L-lysine)-graft-dextran, but not poly(L-lysine) homopolymer or a basic peptide, induces the B–Z transition of poly(dG–dC) · poly(dG–dC). At a cationic amino group concentration of 10−4 M the copolymer stabilizes Z–DNA. The transition pathway from the B to the Z form is different to that observed previously. We speculate that the cationic backbone of the copolymer, which reduces electrostatic repulsion among DNA phosphate groups, and the hydrophilic dextran chains, which reduce activity of water, cooperate to induce the B–Z transition. The copolymer specifically modified the micro-environment around DNA molecules to induce Z–DNA formation through stable and spontaneous inter-polyelectrolyte complex formation.