This study presents a novel coupling design with adjustable torsional stiffness. Since the torsional stiffness of the coupling can be adjusted, it can potentially be applied to tune the torsional natural frequencies of rotating systems. The presented coupling design provides clear benefits compared to typical flexible element couplings. For applications with typical flexible element couplings with elastomeric inserts, the torsional stiffness can be adjusted by changing the elastomers. However, this often requires disassembly of the coupling, and the torsional stiffness adjustment has to be performed in large increments. In the presented coupling design, the torsional stiffness can be adjusted without disassembly. The torsional stiffness can be tuned to a wide range of stiffnesses and with arbitrarily small increments. The torsional stiffness of the coupling was determined by analytical calculations, FEM simulations and experimentally. In the experimental tests, the torsional stiffness range of the coupling was measured to be between 8–126 kNm/rad. Experimental measurements agree with the calculated and simulated stiffness values. The coupling design was considered to be successful, since the study confirmed that the torsional stiffness of the coupling can be adjusted to a wide range of different values.