A single crystalline InSb nanosheet is an emerging planar semiconductor material with potential applications in electronics, infrared optoelectronics, spintronics, and topological quantum computing. Here, we report on the realization of a quantum dot device from a single crystalline InSb nanosheet grown by molecular-beam epitaxy. The device is fabricated from the nanosheet on a Si/SiO2 substrate, and quantum dot confinement is achieved by the top gate technique. Transport measurements of the device are carried out at a low temperature in a dilution refrigerator. It is found that the measured charge stability diagram is characterized by a series of small Coulomb diamonds at high plunger gate voltages and a series of large Coulomb diamonds at low plunger gate voltages, demonstrating the formation of a gate-tunable quantum dot in the InSb nanosheet. Gate-defined planar InSb quantum dots offer a renewed platform for developing semiconductor-based quantum computation technology.