In this work, we investigate the formation and structure of complexes between quaternized poly(2-(dimethylamino)ethyl methacrylate-b-lauryl methacrylate-b-(oligo ethylene glycol) methacrylate) (QPDMAEMA-b-PLMA-b-POEGMA) amphiphilic triblock terpolymer micelles, with different compositions, and DNAs of different lengths. In aqueous solutions, the terpolymers self-assemble in spherical micelles comprising hydrophobic PLMA cores and mixed QPDMAEMA/POEGMA coronas. Terpolymer/DNA micelleplexes were prepared in a wide range of amine over phosphate group ratios (N/P), and linear DNA of different lengths (2000 base pairs and 113 base pairs) was utilized. The complexation ability of the polymeric micelles with DNA was studied by UV-vis spectroscopy and ethidium bromide quenching assays using fluorescence spectroscopy. The thermodynamic interactions that take place during complexation were investigated by isothermal titration calorimetry, whereas the structural characteristics of the micelleplexes were determined by lights-cattering techniques (dynamic, static, and electrophoretic) and cryogenic transmission electron microscopy (cryo-TEM) imaging. All terpolymer micelles effectively complex DNAs, forming micelleplexes, with their size, surface charge, and structure depending on the ratio of the components, length of the DNA, and the chemical structure of the terpolymers. Cryo-TEM revealed the clustering of terpolymer micelles due to complexation with the clusters having different topologies influenced remarkably by the length of the DNA utilized.