African mahogany (Khaya senegalensis) is a good option for the growing demand of the furniture industry since it is resistant to Hypsipyla grandella Zeller, a borrowing insect which causes damage to plants of the genus Swietenia in the American continent. The objective of this paper was to verify the effects of potassium doses on the growth of young African mahogany plants growing under two hydric conditions. The doses consisted of 0, 50, 100, 150, and 200 mg dm-3 of K in combination with hydric conditions of 30 and 70% of the soil maximum water holding capacity in pot, the treatment replications being distributed according with a completely random design and a 5 × 2 factorial scheme with 5 replications. At 180 days, the plants showed higher efficiency of potassium use, stomata conductance, transpiration, photosynthesis, height, diameter, leaf area, root dry matter, stem, and leaves at 70% of moisture. At the moisture of 30%, the potassium doses increased leaf area and leaf dry matter. Maximum water use efficiency was attained at 71 mg dm-3 of K. At 70% of soil moisture, increasing doses of K caused the reduction in N concentration in the leaves and a concomitant increment in phosphorus and potassium concentrations. Young African mahogany plants do not demand high doses of K as long as the other mineral necessities are met. Under hydric restriction, potassium increases the efficiency of water use and leaf biomass. Key words: Khaya senegalensis, argisol, hydric restriction, potassium fertilization, photosynthesis.