The invasive cyanobacterium, Cylindrospermopsis raciborskii, was cultured semicontinuously for 18 days at different carbon and phosphorus levels. Growth and photosynthesis were measured to study the combined effects of CO2 and phosphorus availabilities. The results showed that enrichment of CO2 or inorganic phosphorus (Pi) levels in the culture medium significantly increased specific growth rate (μ), light-saturated photosynthesis (Pm) and photosynthetic efficiency (α), but tended to decrease dark respiratory rate (Rd), saturating irradiance for photosynthesis (Ek) and the light compensation point (Ec). However, photosynthetic responses to inorganic carbon concentration in C. raciborskii were affected by interactions between CO2 and Pi. At high Pi concentration, elevated CO2 significantly increased the inorganic carbon-saturated photosynthesis rate (Vmax) and substrate concentration giving half-maximal photosynthetic rate [K0.5 (DIC)], whereas at low Pi levels, elevated CO2 slightly decreased the values of Vmax and K0.5 (DIC). Additionally, enrichment with Pi significantly increased K0.5 (DIC) and Vmax regardless of the CO2 level, suggesting that Pi plays a role in photosynthetic carbon fixation in C. raciborskii. Furthermore, when C. raciborskii was batch cultured in different carbon and phosphorus concentrations for 72 h, the uptake rate of nitrogen increased with enrichment of both CO2 and Pi, while the uptake rate of phosphorus was enhanced only by increases in Pi concentration. The results suggest that the availability of CO2 and phosphorus have combined and independent effects on photosynthesis and nutrient uptake in C. raciborskii, reflecting that changes in environmental factors could influence the dominance of this invasive cyanobacterium. These combined effects need to be considered when generalizing this species response to changing CO2 levels.