High-fidelity entangling gates are useful for circuit-based quantum computing, yet their realization in a scalable platform is challenging. Optically trapped ultracold polar molecules offer a potentially scalable platform. Here, we introduce a two-qubit molecular entangling gate via a novel ancilla-assisted dark-state mechanism, where high fidelity is achievable. In particular, we find that by using appropriate qubit states for the ancilla and the data qubits, dark states can appear, and entanglement can emerge by driving the ancilla only. It is also feasible to extend the two-qubit gate to a multi-qubit gate via the same dark-state mechanism.