Abstract One of the main challenges in pepper production is to enhance seed germination energy and germination, and to grow healthy nursery plants with strong root system. Trichoderma species colonize roots as they grow and provide season-long benefits to plants, which is why Trichoderma species are widely used as plant growth promoter agents and promoters of plant defence mechanisms. This study evaluated the effectiveness of seed biopriming with Trichoderma isolates for growth promotion of pepper plants in early stage and their effects on seedling physiology. Nine out of ten Trichoderma isolates positively affected root weight of pepper seedlings, while three out of ten positively affected shoot weight. Root and shoot lengths were mainly unaffected. Germination energy was positively affected by five isolates with up to 40% increase compared to the control, while germination was significantly enhanced by two isolates with up to 22% increase. Considering seedling physiology, two different strain-dependent modes of actions were expressed. Promising Trichoderma isolates induced formation and accumulation of reactive oxygen species (ROS) which acted as signal molecules that increased germination energy and germination. Positive correlation was found between pyrogallol peroxidase, superoxide dismutase, catalase activity and germination in plants treated with these isolates.