In recent years, there has been notable progress in the preparation and control of ultracold gases of diatomic molecules. The next experimental challenge is the production of ultracold polyatomic molecular gases. Here, we report the creation of an ultracold gas of 23 Na 40 K 2 triatomic molecules from a mixture of ground-state sodium-23-potassium-40 ( 23 Na 40 K) molecules and potassium-40 ( 40 K) atoms. The triatomic molecules were created by adiabatic magneto-association through an atom-diatomic molecule Feshbach resonance. We obtained clear evidence for the creation of triatomic molecules by directly detecting them using radio-frequency dissociation. Approximately 4000 triatomic molecules with a high-peak phase-space density of 0.05 could be created. The ultracold triatomic molecules can serve as a launchpad to probe the three-body potential energy surface and may be used to prepare quantum degenerate triatomic molecular gases.