Plants are commonly exposed to fluctuating illumination under natural light conditions, causing dynamic photosynthesis and further affecting plant growth and productivity. In this context, although the vital role of potassium (K) in steady-state photosynthesis has been well-established, knowledge of the dynamic changes in photosynthesis mediated by K remains scarce. Here, the gas-exchange and chlorophyll fluorescence parameters under steady-state and dynamic photosynthetic responses were quantified in Phaseolus vulgaris L. seedlings grown under K-deficient (−K, 0.02 mM K) and normal K (+K, 2 mM K) conditions. After a transition from low to high light, the time course–induction curves of the net photosynthetic rate (A), stomatal conductance (g_s), mesophyll conductance (g_m), and maximum carboxylation rate (V_cmax) showed an obvious decline in the −K treatment. In comparison with the +K treatment, however, there were no statistical differences in the initial A and V_cmax values in P. vulgaris supplied with deficient K, suggesting that the K-deficiency-induced decreases in A and V_cmax were light-dependent. Interestingly, the time to reach 90% of the maximum A, g_s, and g_m significantly decreased in the −K treatment in comparison with the +K treatment by 27.2%, 45.6%, and 52.9%, respectively, whereas the time to reach 90% of the maximum V_cmax was correspondingly delayed by almost two-fold. The photosynthetic limitation during the induction revealed that the biochemical limitation was the dominating factor that constrained A under the −K conditions, while, under the +K conditions, the main limiting factor changed from biochemical limitation to stomatal limitation over time. Moreover, g_m imposed the smallest limitation on A during induction in both K treatments. These results indicate that a decreased K supply decreases the photosynthetic performance under fluctuating light in P. vulgaris and that improving the induction responses of biochemical components (i.e., V_cmax) has the potential to enhance the growth and productivity of crops grown in K-poor soil.