•Low and high temperature decreased shoot fresh weight.•Se and N-Se can improve select tomato plant growth parameters after a short-term pulse of high and/or low temperature stress.•A low level of Se applied at high and low temperature stress promoted shoot and root growth.•Se is more effective in alleviating the deleterious effect of high/low temperature stress than N-Se. High and low temperatures are environmental stresses that can damage agricultural crops and limit their distribution and yield. The objective of this study was to investigate the effect of selenium (Se) and nano-selenium (N-Se) on tomato (Lycopersicum esculentum Mill. cv. ‘Halil’) plant growth in response to high and low temperature stress since Se is a well-known stress moderator. Se was applied at 0μM (control), 2.5μM (Se1), 5μM (Se2) and 8μM (Se3). Similarly, N-Se was applied at 1μM (N-Se1), 4μM (N-Se2), 8μM (N-Se3) and 12μM (N-Se4). All plants were exposed to optimal temperature (25/17±2°C day/night=T1), then to high (24h at 40°C=T2) and low (24h at 10°C=T3) temperature stress, then placed at optimal conditions for 10 days. Se, when applied at 2.5μM, promoted plant growth after both high and low temperature stress were applied. More specifically, shoot dry weight and diameter increased in T2, shoot fresh weight, dry weight and diameter increased in T3, and root fresh weight, dry weight and root volume increased in T2 and T3. N-Se had no effect on plant growth. In T3, Se1 increased the chlorophyll content of leaves by 19.2% while N-Se1 improved it by 27.5%. Root volume increased 33.3% after treatment with Se1 in T2 and by 60% in T3. Se and N-Se increased relative water content after T2 and T3 stress. Thus, Se and N-Se can improve select tomato plant growth parameters after a short-term pulse of high and/or low temperature stress.