The kinetics of Diels‐Alder (DA) reactions in water has been known to be altered by salts for a long time. Yet the question how salts influence the reaction rate, either as rate‐enhancing or rate‐reducing additives, has so far remained unresolved. Conflicting hypotheses involve (i) indirect salt contributions through the modulation of internal pressure and (ii) making (or breaking) of the so‐called “water‐structure” by salts that strengthen (or weaken) the hydrophobic effect. In contrast to the qualitative nature of these hypotheses, here we answer this question quantitatively through a combination of transition state theory and fluctuation adsorption‐solvation theory (FAST) using the DA reaction between anthracene‐9‐carbinol and N‐ethylmaleimide as an example. We show that rate enhancement is driven by the salting out of the hydrophobic reactant, while rate‐enhancing salts exhibit stronger affinity to the transition state.