Quantum-dot Cellular Automata (CA) is a suitable nanotechnology for replacing CMOS circuits. The majority, inverter, and reversible structures, are the primary building blocks in quantum-dot CA circuits. Since the reversible gates have recently attained great attention, this paper proposes a new design of a reversible building block based on the Feynman gate. The proposed quantum-dot CA-based Feynman gate is denser and has low delay than the existing circuits. This paper also tests the possible implementation of the proposed design in quantum-dot CA. The QCADesigner is employed to test the functionality of the proposed structure. The proposed efficient quantum-dot CA-based Feynman gate has only 36 cells in three-layer. It consumes 0.03 µm2, which is the smallest among all previous designs. A quantum-cost comparison of the proposed reversible Feynman gate with conventional reversible gates demonstrates the cost-effectiveness of quantum-dot CA. The simulation result matched the truth table of the Feynman gate, indicating that the suggested quantum-dot CA architecture is functional. The proposed architecture can also be used for n-bit code converters as well.