Abstract: Background: This study is about an assessment of plant growth promoter and biocontrol properties of Trichoderma harzianum in onion. Two isolates of T. harzianum one originating from Eco-Tâ (South Africa) and the other from a onion field soil (Burkina Faso) were evaluated for their use as onion growth promoters and their efficiency in protecting onion seedlings against damping-off caused by Fusarium oxysporum and F. solani. Materials and Methods: Trichoderma harzianum was used for coating onion seeds with a conidial suspension concentrated to 107 spores mL–1 and for directly spraying onion seedbeds with the same conidial suspension. Sowing was done with coated seeds in trays containing a doubly sterilized substrate inoculated or not with Fusarium strains. Non-coated seeds were sowed in the same substrate sprayed with T. harzianum conidial suspension and inoculated or not with Fusarium strains. All trays were kept in greenhouse for 1 month. Results: Coating the seeds and spraying the seedbeds with the conidial suspension of the two T. harzianum isolates led to a significant increase in the number of living seedlings, seedling length, root length, number of leaves and fresh weight of seedlings 30 days after sowing in sterilized soils. Sowing the T. harzianum coated seeds in soils inoculated with Fusarium led to a significant reduction in seedling damping-off due to the pathogens and improved the growth parameters of seedlings compared with non-coated seeds. Spraying the conidial suspension of T. harzianum isolates on seedbeds inoculated with both Fusarium strains produced the same results, but at a lower intensity than was the case with seed coating. Conclusion: These results reveal that the use of an indigenous isolate of T. harzianum originated from Burkina Faso could be a novel biocontrol strategy against the damping-off of onion seeds in an environmentally sustainable way. This study discusses further development of a simple and accessible method for the mass production and conservation of T. harzianum and the search for an effective and inexpensive sticker for seed coating. An isolate of T. harzianum native to Burkina Faso was shown to improve the germination of onion seeds and to protect seedlings against Fusarium damping-off. This isolate can be exploited in the development of integrated onion crop protection in West Africa.