A hyperbranched polymer (HBP) made of three-way junction (TWJ) DNAs is reported. Three types of 26-mer DNAs with 5'-ends modified with psoralen (PSN) were synthesized. All had self-complementary sequences starting from the 5'-end to the sixth base (AAGCTT), allowing intermolecular hybridization. The base sequences of the remaining 20-mer sites were designed so that upon hybridization, three strands had a TWJ structure with a mass of 25,000 that could be further grown by forming HBPs. PSN photochemically reacts to form interstrand cross-links that increase the polymer stability. Aggregates [(380 ± 44) nm and (65 ± 6) nm] detected with dynamic light scattering for TWJ-DNA solutions were also imaged by electron microscopy and atomic force microscopy, providing evidence of hyperbranched polymerization. The TWJ unit also polymerized on solid substrates such as Au and glass and formed self-assembled monolayers (SAMs). The HBP SAMs were integrated into commercial Pt-interdigitated electrode arrays. The DNA devices had current-voltage curves typical of metal-insulator-metal Schottky diodes; the effective barrier heights and the ideality factors were 0.52 ± 0.002 eV and 21 ± 3.2, respectively. The series resistances were (26 ± 3.3) × 106 Ω, which may provide insights into DNA electron transport. The DNA HBP enables stable electrical connections with probe electrodes and will be an important single-molecule platform.