Drought and salinity are the primary constraints for barley production in arid and semi-arid regions. Although potassium (K) fertilization and biochar amendment each individually mitigate these stresses, their interactive effects under combined drought and salinity conditions remain poorly characterized for barley. A four-factor greenhouse pot experiment was conducted using a completely randomized factorial design comprising three drought levels (D0: 100%, D1: 75%, D2: 50% field capacity), three K rates (K0: 0, K1: 160, K2: 320 kg K₂SO₄ ha⁻¹), two biochar rates (B0: 0, B1: 18 t ha⁻¹ of olive mill waste biochar), and two soil types (non-saline, NS: EC = 0.37 dS m⁻¹; saline, S: EC = 6.23 dS m⁻¹), yielding 36 treatment combinations replicated four times (144 pots). Response variables included shoot length, shoot and root fresh and dry weights, chlorophyll index (SPAD), and leaf macronutrient concentrations (N, P, K, Ca, Mg) and soil organic matter. Severe drought (D2) reduced shoot fresh weight by 44% and root fresh weight by 36% in non-saline soil without amendments. The K1 × B1 combination recovered 83% of the well-watered, unamended shoot fresh weight under D2 in non-saline soil and performed consistently as the superior treatment under saline conditions. The K2 rate provided no consistent advantage over K1 and reduced root dry weight in several combinations, suggesting ionic imbalance at supraoptimal supply. SPAD values were not significantly affected by drought, indicating chlorophyll stability in this cultivar under the tested stress range. All four-way interactions (D × K × B × S) were statistically significant for most morphological and nutritional variables (p < 0.01). Co-application of biochar (18 t ha⁻¹) with K fertilization at 160 kg ha⁻¹ is an effective strategy to maintain vegetative growth performance in water-limited, saline-prone environments. Field-scale validation including grain yield and water-use efficiency measurements is necessary before definitive agronomic recommendations can be made.