The twist angle in van der Waals heterostructures enables continuous tuning of electronic states, yet its influence on superconductivity has been difficult to access under in situ control. Here, we demonstrate in situ manipulation of the twist angle in a two-dimensional superconductor using a scanning tunneling microscope (STM) tip, which simultaneously probes its superconducting properties. Monolayer NbSe2 islands were grown on graphene by molecular beam epitaxy and mechanically rotated via STM tip. Ultralow-temperature tunneling spectroscopy below 100 mK reveals that the superconducting gap exhibits a sixfold angular dependence and varies continuously by a factor of two. The extrema of the gap are found at twist angles where the Fermi surface of graphene aligns with regions of the NbSe2 Fermi surface near the Γ and M points, suggesting a relation to Fermi surface overlap in momentum space, as discussed previously (M. Naritsuka et al., Nat. Phys. 21, 746 (2025)). These results establish a platform for continuous twist control of superconductivity and provide a route toward systematic exploration of twist-engineered quantum states.