The genus Petunia is a plant of high economic importance in the world-wide horticulture. These ornamental plants are often exposed to soil salinity that negatively affects their development. Chitosan is a biopolymer with multiple applications in plant breeding but it also minimizes the adverse effects of abiotic stresses on plant growth. The objective of this study was to investigate the effects of chitosan on petunia shoots development under salt stress in vitro. In the first experiment, four types of chitosan with molecular weight of 3.33, 8, 10 and 970 kDa in the concentrations of 0, 10, 15 and 20 ppm were supplemented into MS medium. In the second experiment, petunia shoots were grown on MS medium with the addition of different molecular weight of chitosan in the concentration of 15 ppm each and 100 mM NaCl. The results indicated that all of chitosan types and concentrations stimulate the plant growth in comparison to control. However, 15 ppm chitosan concentration was more effective than other concentrations used. Salinity caused a significant reduction in shoot and root length, fresh and dry mass, plant water contents, while chitosan (970 kDa) adjusted the salt toxicity. It is concluded that chitosan would be able to stimulate the growth of petunia shoots in vitro independent of their molecular weight. It was observed that the addition of chitosan of 970 kDa to MS medium under salinity conditions may alleviate the inhibitory effect of salt stress on the plant growth.