While there is growing interest in understanding how marine life will respond to future ocean acidification, many coastal ecosystems currently experience intense acidification in response to upwelling, eutrophication, or riverine discharge. Such acidification can be inhibitory to calcifying animals, but less is known regarding how non-calcifying macroalgae may respond to elevated CO₂. Here, we report on experiments performed during summer through fall with North Atlantic populations of Gracilaria and Ulva that were grown in situ within a mesotrophic estuary (Shinnecock Bay, NY, USA) or exposed to normal and elevated, but environmentally realistic, levels of pCO₂ and/or nutrients (nitrogen and phosphorus). In nearly all experiments, the growth rates of Gracilaria were significantly increased by an average of 70% beyond in situ and control conditions when exposed to elevated levels of pCO₂ (p<0.05), but were unaffected by nutrient enrichment. In contrast, the growth response of Ulva was more complex as this alga experienced significantly (p<0.05) increased growth rates in response to both elevated pCO₂ and elevated nutrients and, in two cases, pCO₂ and nutrients interacted to provide a synergistically enhanced growth rate for Ulva. Across all experiments, elevated pCO₂ significantly increased Ulva growth rates by 30% (p<0.05), while the response to nutrients was smaller (p>0.05). The δ¹³C content of both Gracilaria and Ulva decreased two-to-three fold when grown under elevated pCO₂ (p<0.001) and mixing models demonstrated these macroalgae experienced a physiological shift from near exclusive use of HCO₃^– to primarily CO₂ use when exposed to elevated pCO₂. This shift in carbon use coupled with significantly increased growth in response to elevated pCO₂ suggests that photosynthesis of these algae was limited by their inorganic carbon supply. Given that eutrophication can yield elevated levels of pCO₂, this study suggests that the overgrowth of macroalgae in eutrophic estuaries can be directly promoted by acidification, a process that will intensify in the coming decades.