Arbuscular mycorrhizal fungi (AMF) and biochar may improve crop growth in drought-prone sandy soils, but their interactive effects under different water regimes remain unclear. We investigated their effects on potato growth and nutrient uptake under water deficit. A semi-field rainout-shelter experiment was conducted in Denmark using low-phosphorus (P) sandy soil (Olsen P: 18.3 mg kg⁻¹), unamended (BC−) or amended with wheat-straw biochar (BC+). Potato plants were grown without (M−) or with Rhizophagus irregularis inoculation (M+) and irrigated at three soil water thresholds (75, 50, and 25% of field capacity). Biochar increased P and nitrogen (N) uptake and tuber biomass, with biomass gains strongly correlated with nutrient uptake. Under high irrigation, BC+M+ increased tuber biomass, P uptake, and N uptake by 55, 73, and 32%, respectively, compared with BC−M−. Without biochar, AMF did not increase nutrient uptake and reduced tuber biomass. AMF root colonization and its soil lipid biomarker were affected by water deficit, with responses also associated with soil pH and P availability. Overall, adequate water availability was critical for realizing the benefits of combined biochar and AMF application in low-P sandy soil.