AbstractC4 plants are expected to have faster stomatal movements than C3 species because they tend to have smaller guard cells. However, little is known about how the evolution of C4 photosynthesis influences stomatal dynamics in relation to guard cell size and environmental factors. We studied photosynthetically diverse populations of the grass Alloteropsis semialata, showing that the origin of C4 photosynthesis in this species was associated with a shortening of stomatal guard and subsidiary cells. However, for a given cell size, C4 and C3–C4 intermediate individuals had similar or slower light‐induced stomatal opening speeds than C3 individuals. Conversely, when exposed to decreasing light, stomata in C4 plants closed as fast as those in non‐C4 plants. Polyploid formation in some C4 plants led to larger stomatal cells and was associated with slower stomatal opening. Conversely, diversification of C4 diploid plants into wetter environments was associated with an acceleration of stomatal opening. Overall, there was significant relationship between light‐saturated photosynthesis and stomatal opening speed in the C4 plants, implying that photosynthetic energy production was limiting for stomatal opening. Stomatal dynamics in this wild grass therefore arise from the evolving interplay between photosynthetic physiology and the size and biochemical function of stomatal complexes.