Beneficial plant-microbe interactions in the rhizosphere are primary determinants of plant health and soil fertility. Some antagonistic fungi have shown great effects toward the growth of plant crops. In this study, two major crops, cotton and potato, were selected to evaluate their growth promotion by the antagonistic fungus Talaromyces flavus. For each plant, five T. flavus isolates were selected from our fungal collection which had shown the highest antagonistic activities against the causal agent of wilt diseases on these plants. In the next step, for every crop, five isolates were used under greenhouse conditions. For evaluation of the plant growth promotion ability of T. flavus isolates, a split-plot trial was arranged in a randomized complete block design with four replications. The main factor was the method of application of T. flavus as a soil treatment, a seed treatment, and a combination of both methods. The subfactor was the use of different fungal isolates. Measured parameters were root length, crown length, plant height, plant fresh weight, and plant dry weight. Results showed that the maximum increase in the above parameters was mediated by the seed treatment method. The most effective isolate for cotton plants was TF-Co-M-23, which increased root length, plant height, plant fresh weight, and plant dry weight by 1.80-, 2.26-, 1.23-, and 1.19-fold, respectively. There were no significant differences among the various treatments affected by T. flavus in terms of crown length. The most effective isolate for potato plants was TF-Po-V-50, which increased root length, crown length, plant height, and plant dry weight by 1.71-, 1.09-, 1.45-, and 3.75-fold, respectively. The overall results of this study suggest that it may be possible to promote cotton and potato growth characteristics by using the antagonistic fungus T. flavus.