Although optimal control has been used extensively for greenhouse climate management, its application to plant factories is still in its infancy. In this paper, profitability of growing lettuce in a Chinese plant factory is investigated by means of optimal control computations. To that end, first, a lettuce growth model is adapted to fit a plant factory environment. Next, this model is calibrated and validated using nine sets of experimental data with different LED light intensities. Using the calibrated and validated model, optimal control computations are used to produce a 3D plot revealing the influence of the electricity and lettuce price on maximum profit. Lettuce's physiological demand for dark periods during artificial lighting is incorporated by fixing this dark period to eight hours a day. Therefore, the optimal LED light intensity pattern obtained from the optimal control computations could be used in the actual production process. Maximum profit can reach 264.88 RMB m⁻² assuming a Chinese plant factory fresh lettuce price of 34.5 RMB kg⁻¹. When lettuce must be sold at 5.01 RMB kg⁻¹, which represents the price for an open field product, profit always comes out negative. Besides, LED lighting is not advised when the electricity price is greater than 0.84 RMB kWh⁻¹ under these circumstances. Profit is only positive when the lettuce price is over 20 RMB kg⁻¹.