Sites in need of restoration typically have one or more environmental factors that limit seedling establishment. Identifying ecophysiological responses to environmental stressors can provide important insights into mitigating measures that would allow seedlings to overcome such constraints to survival. Koa (Acacia koa A. Gray) is a nitrogen-fixing tree species endemic to Hawaiʻi that is highly valued in restoring degraded forest ecosystems, which are often limited in available water and phosphorus. This study examined how koa seedlings respond to conditions of reduced water (65 W) and no phosphorus (0P). After 17 weeks, seedlings subjected to 65 W or 0P accumulated less biomass, smaller root-collar diameters, and lower nitrogen and phosphorus contents. Combined reductions in water and P resulted in seedlings with increased root to shoot dry biomass and shorter shoots. Seedlings subjected to 65 W also had lower instantaneous rates of CO₂ assimilation, but higher instantaneous water-use efficiencies following irrigation, suggesting that koa responds to water deficits by decreasing water loss via reduced stomatal conductance. Seedlings subjected to 0P had similar rates of CO₂ assimilation relative to those grown with adequate P, suggesting that koa is able to employ strategies to avoid physiological impairment from conditions of inadequate P. Future research should assess whether subjecting koa seedlings to reduced water before planting on water-limited sites cues increased drought resistance and whether uptake and storage of P by seedlings in the nursery better supports growth following outplanting, particularly on sites with anticipated low plant-available water.