Plant biostimulants can enhance crop nutrition status, stress tolerance, yield and quality in an environment-friendly manner. The aim of this study was to determine the effect of algae and bacteria preparations on the yield and nutritional parameters of leaf and romaine lettuce cultivated for spring and summer crop. The application of a combined biostimulant consisting of plant growth-promoting bacteria (Bacillus licheniformis, Bacillus megatherium, Azotobacter sp., Azospirillum sp., and Herbaspirillum sp.) and freshwater algae (Chlorella vulgaris) was done by watering the lettuce every 14 days, and a determination of the fresh weight, total antioxidant capacity, and total carotenoids content were performed. The result revealed that the application of bacterial-algal preparation significantly affected the plant weight of both romaine and leaf lettuce in the spring and summer seasons. The highest increase in the weight of romaine lettuce reached 18.9% in the spring crop, while in the case of leaf lettuce, biostimulant treatment led to a 22.7% higher weight in the summer crop. Total antioxidant capacity and total carotenoids content showed increased values in the summer crop of romaine lettuce, while for the leaf lettuce there were no differences between treatments. Therefore, the positive effect of bacterial-algal treatment on lettuce yield, total antioxidant capacity and total carotenoids confirm that it could be applied for improving romaine lettuce yield quality and quantity, especially in stress, summer conditions.