Supplemental lighting and CO2 enrichment have been employed to promote plant growth in commercial plant production in greenhouses. In a semi-closed plant production system with a large number of plants at a high density, the relative humidity in the air around growing plants could be in excess of 80%. This research was initiated to determine the effects of CO2 concentration and photoperiod on the growth of plants under relatively high humidity conditions. In the experiment, lettuce plants were grown for 13 days under eight combinations of two CO2 levels (CO2, 0.38 and 0.76 mmol·mol-1), two photoperiods (PP, 16 and 24 h/day), and two relative humidity levels (RH, 80% and 90%) in growth chambers. The air temperature was 25 °C. Plants were illuminated with fluorescent lamps at a photosynthetic photon flux of 0.23 mmol·m-2·s-1. The dry mass of lettuce shoots (leaves and stems) grown in 0.76 mmol·mol-1 CO2, 24 h/day PP, and 80% to 90% RH was greatest in all treatments and was five times the least value obtained in 0.38 mmol·mol-1 CO2, 16 h/day PP and 90% RH. The dry mass of lettuce shoots decreased to 40% as RH increased from 80% to 90 % under 0.38-0.76 mmol·mol-1 CO2 and 16 h/day PP. Growth suppression by excess humidity was less significant in longer PP and higher CO2. Supplemental lighting and CO2 enrichment would be more effective for promoting growth of plants grown under higher humidity conditions.