Partial root zone drying (PRD) is a new water-saving irrigation strategy being tested in many crop species. Until now it has not been investigated in potatoes (Solanum tuberosum L.). A field experiment on sandy soil in Denmark was conducted under a mobile rainout shelter to study effects of two subsurface drip irrigation treatments ((1) Full Irrigation (FI) receiving 100% of evaporative demand; and (2) PRD receiving 70% water of FI) on potato yield, tuber size, leaf water relations and irrigation water use efficiency (IWUE). The PRD treatment was started just after the end of tuber initiation for two months during tuber bulking and maturing stage and was shifted from one side to the other side of the plants every 5-10 days when FI plants had used 20-25 mm. Compared to FI plants, stomatal conductance was generally lower in the PRD-treated plants, whereas leaf water potential tended to be lower on only a few days. No significant difference was found between the treatments in tuber yield. IWUE was 61% higher in the PRD than in FI treatment. We conclude that reduction of soil water content under PRD induced partial stomatal closure. The effect likely contributed to water saving under PRD irrigation. The reasons for a better tuber size distribution caused by PRD, however, remain elusive. For optimizing PRD irrigation, the crop physiological reactions to shifting intervals and level of irrigation reduction should be further studied at different growth stages.