In aqueous media, negative photochromism can occur from cationic, protonated merocyanines (MCH+) to spiropyrans (SP) upon exposure to visible light. Herein, we demonstrate the uniquely simple manner in which polymers can tune the key properties of these photochromes through copolymer preparation. Nine water-soluble, SP-containing polymers yield wide ranges of key properties for negative photochromism-accessible MCH+ pK(a) values spanning 5 pH units, quantum yields of photochemical ring closure up to 0.2, and rates of thermal equilibration ranging 2 orders of magnitude. In buffered solutions, many of these polymers showed photostationary states with greater than 80% conversion to SP and no detectable photochromic fatigue over 15 cycles. These results demonstrate the potential for tuning key properties of the MCH+ photochrome through both the spiropyran pendants themselves and the water-solubilizing comonomers. The dependence of MCH+ pK(a) on comonomer structure enabled tunable range of pH for reversible photoacid activity using a single spiropyran derivative, further showcasing the versatility of water-soluble photochromic polymers.