There is a very narrow margin in selenium deficiency and toxicity although it is an important element for humans, animals, and plants. Effects of selenium (Se) on the growth and physiomorphological parameters in maize were studied grown in soil spiked with sodium selenate (Na2SeO4) in 5 different concentrations (i.e., 0. 2.5, 5.0, 10.0, and 20.0 mg kg−1). The growth of plants was affected by high Se concentration. However, maximum increases in plant height and root length were observed at low Se (2.5 mg kg−1) which were 17.89 and 23.17%, respectively. At higher Se concentrations (20 mg kg−1), a considerable reduction was observed in dry matter, root length, antioxidant enzymes, and other physiological parameters. The dry matter of plants was also analyzed for nutrient (Fe and Zn) concentrations. Results indicated that Se stress inhibits plant growth. Gas exchange parameters were also found to be decreased under stress conditions, but at a lower Se level (2.5 mg kg−1), improvement in transpiration rate (63.46%), photosynthetic rate (47.47%), and stomatal conductance (54.55%) was observed. The reduction in growth attributes may be due to the high accumulation of Se in roots and the disturbance in gas exchange parameters. However, the principal component analysis revealed that higher Se levels were more hazardous for maize growth and physiological responses as compared to low Se levels.