A ternary logic system to realize the simplest multivalued logic architecture can enhance energy efficiency compared to a binary logic system by reducing the number of transistors and interconnections. For the ternary logic system, a ternary logic device to harness three stable states is needed. In this study, a vertically integrated complementary metal–oxide–semiconductor ternary logic device is demonstrated by monolithically integrating a thin-film transistor (TFT) over a transistor-based threshold switch (TTS). Because the TFT and the TTS have their own source (S), drain (D), and gate (G), there are physically six electrodes. But the hybrid ternary logic device of the TFT over the TTS has only four electrodes: S, D, GTFT, and GTTS like a single MOSFET. It is because the D of the underlying TTS is electrically tied with the S of the superjacent TFT. By combining an on- and off-state of the TFT and the TTS, ternary logic values of low current (“0”-state), middle current (“1”-state), and high current (“2”-state) are realized. Particularly, static power consumption at the “1”-state is decreased by employing the TTS with low off-state leakage current compared to previously reported other ternary logic devices. In addition, a footprint of the ternary logic device with the vertically overlaying structure that has a framework of “one over the other” can be lowered by roughly twice compared to that with the laterally deployed structure that has an organization of “one alongside the other”.