Lithium-sulfur batteries are limited by the high mobility of polysulfides in the electrolyte, which allows them to migrate from the cathode to the lithium anode. This is known as polysulfide shuttling and simultaneously diminishes the active material and poisons the anode. Various cathode additives have been shown empirically to mitigate this problem, although the mechanism is not often ascertained experimentally. Herein, we demonstrate for the first time that the small molecule additive caffeine reduces the polysulfide shuttling current in a lithium-sulfur battery and decreases the capacity fade in galvanostatic cycling experiments. Using in-operando Fourier transform infrared (FTIR) spectroscopy, we identify reversible shifts in the carbonyl stretching frequencies of caffeine that are strongly correlated with the onset of polysulfide formation during both discharging and charging. These spectroscopic shifts are consistent with a polar-polar interaction between polysulfides and the carbonyl groups of caffeine, leading to the observed decrease in polysulfide mobility.