Water stress is a major limiting factor for plant growth and development. In this study, we investigated the effects of L-β-phenyllactic acid (LPA) on growth of rice (Oryza sativa L.) seedlings under polyethylene glycol (PEG)-induced water deficit conditions. Seedlings were culture at 30 °C for 14 days in growth pouches supplemented with 1/100-strength Murashige and Skoog (MS) medium and PEG in the presence or absence of 100 mg L^-1 LPA. As evidenced by plant height, LPA application enhanced seedling growth under PEG-induced water deficit by 13%. The shoot dry weight was slightly increased, whereas that of roots was markedly enhanced during LPA treatment by 26% under water-deficit conditions. No difference was observed among treatments in the number of roots per seedling. The ratio of shoot dry weight to shoot length (RWL) was constant regardless of treatment, indicating that LPA does not cause spindly shoot growth. The total length, surface area, and volume of fine roots were increased by LPA under PEG-induced water deficit conditions. Plant height was significantly correlated with total root surface area and volume. The results imply that PEG-induced water deficit in rice seedlings can be alleviated by LPA application. This alleviative effect is partially attributable to alterations in root system developmental patterns, with increases in fine root total length, surface, and volume accelerating water and nutrient acquisition from the culture medium.