The high concentrations of dissolved salts in irrigation water represent one of the main challenges for agricultural production in the semiarid region of Northeast Brazil. To mitigate the harmful effects of salt stress on plants, it is crucial to adopt efficient strategies, with foliar application of biostimulants standing out. In this context, the objective of this study was to evaluate the effect of foliar application of the seaweed extract Ascophyllum nodosum on gas exchange, photosynthetic pigments, and growth of mini watermelon cv. Sugar Baby under irrigation with saline water. The experiment was conducted using a randomized block design in a 4 x 5 factorial scheme: four concentrations of the A. nodosum nodosum-based biostimulant (0-control, 1.0, 1.5, and 2.0 g L-1) were combined with five levels of electrical conductivity of the irrigation water (ECw) (0.4-control, 1.2, 2.0, 2.8, and 3.6 dS m(-1)), with three replications. Water salinity above 0.4 dS m(-1) reduced the relative water content, photosynthetic pigments, gas exchange, and growth; however, it increased electrolyte leakage in mini watermelon plants. Biostimulant concentrations between 0.9 and 1.0 g L-1 increased leaf area and internal CO2 concentration.