Many wetland restoration projects are initiated with phosphorus (P) retention as a primary objective. While undisturbed wetlands often are net sinks for P and other nutrients, there is evidence that newly flooded restoration wetlands on former agricultural land initially release P to surface waters. The objectives of this study were to: 1) measure P release from soils to overlying surface waters that would occur when re-flooding agricultural fields to restore a lake fringe wetland connected to Upper Klamath Lake, Oregon; and 2) identify management strategies to abate nutrient release from soils during restoration to minimize P loading to Upper Klamath Lake. We simulated the process of re-flooding soils using mesocosms in a laboratory experiment. The soils were flooded with lake water, and the water was replenished on a weekly basis. The net P flux from soils to surface water was estimated by measuring differences in P concentrations between water that had been in the mesocosms and the lake water used for replenishment. After the flooding experiment, we measured the concentrations of four forms of soil P using a modification of the Hedley procedure, to examine relationships between soil P chemistry and P release. The majority of P was released in the first two days of the experiment, and all detectable P was released by the end of the second month. We estimated that 1–9 g P/m² were released from the soils to the water column over the course of the experiment, which amounted to 1%–16% of total soil P. Scaling up to the entire wetland, this totals approximately 64 tons P released over 3,000 ha. We did not find any statistically significant relationships between any of the four forms of soil P and the amount of P released in the flooding experiment. Even though we demonstrate here that P is released while undertaking wetland restoration projects on former agricultural land, it is likely to be a temporary process, and once the wetland begins to resume more natural hydrological and biogeochemical functions and vegetation structure, it will re-start the process of soil accretion and P sequestration.