There has been much interest in I–(H2O) as a simple model for a hydrated iodide ion. Here we explore how this fundamental ion–solvent interaction is modified by the presence of a counterion, specifically Cs+. This has been achieved by forming the CsI­(H2O) complex in superfluid helium nanodroplets and then probing this system using infrared spectroscopy. The complex retains the ionic hydrogen bond between the I– and a water OH group seen in I–(H2O), but the Cs+ ion substantially alters the anion–water interaction through formation of a cyclic Cs+–O–H–I– bonding motif. As with I–(H2O), the OH stretching band derived from the hydrogen-bonded OH group shows substructure, splitting into a clear doublet. However, in contrast to I–(H2O), where a tunneling splitting arising from hydrogen atom exchange plays a role, the doublet we observe is attributed solely to an anharmonic vibrational coupling effect.