We experimentally demonstrate the active control of the coupling strength between porphyrin dyes and surface plasmon polaritons supported by a thin gold layer. This control is externally exerted by a gas flow and is reversible. The hybridized exciton–polariton branches resulting from the exciton–plasmon coupling display a splitting proportional to the coupling strength of the light–matter interaction. The coupled system changes from the weak (no splitting) to the strong coupling regime (splitting of 130 meV) by controlling the effective oscillator strength in the dye layer, via exposure to nitrogen dioxide. The modification of the coupling strength of the system allows tailoring of the dispersion of the hybridized modes as well as of their group velocity.