Leaf blight and spot caused by Stemphylium spp. and Alternaria spp. are the most common destructive tomato diseases in northwestern Algeria. During 2018 growing seasons, more than 30% of samples collected from plants grown in greenhouses or open fields were infected with Stemphylium. Initial symptoms were small, multiple, irregular to oval, yellow leaf spots, which enlarged to brown lesions later. In these lesions, Stemphylium mostly co-occurred with Alternaria spp. Twenty nine Stemphylium isolates were characterized based on morphological features, and multi-locus phylogenies using ITS, gpd, and cmdA genomic loci. Five Stemphylium species (S. lycopersici, S. gracilariae, S. eturmiunum, S. vesicarium, S. lycii) were associated with tomato leaf spot, of which S. lycii is a new report for tomato. Pathogenicity tests on healthy 2-months-old tomato seedlings reproduced symptoms similar to those observed in tomato crops. The tested fungus isolates differed in pathogenicity. Two isolates of S. lycopersici were more aggressive than those of the other species, causing major lesions on tomato plants. The five identified Stemphylium species are reported for the first time as new pathogens for tomato in Algeria, and S. lycopersici, S. gracilariae, S. eturmiunum, and S. lycii as new species of Algerian mycoflora.