Using seawater for mixing concrete is potentially advantageous from a sustainability perspective. However, the presence of high concentrations of chloride in the seawater can lead to corrosion of steel reinforcement. This issue can be addressed by using non-corrosive reinforcement; e.g., fiber reinforced polymer (FRP) bars. Moreover, the global threat of freshwater scarcity suggests that the use of seawater in concrete mixtures becomes plausible in the future. This paper reports on the results of an extensive experimental study to compare the fresh and hardened properties of freshwater- and seawater-mixed concretes. The experimental program included the following tests: (a) characterization of fresh concrete (slump flow, density, yield, air content, and setting time); (b) mechanical characterization of hardened concrete (compressive strength, splitting tensile strength, and shrinkage); and (c) permeability performance of hardened concrete (rapid chloride permeability, chloride migration, and water absorption). The use of seawater had a notable effect on the fresh concrete properties. Mechanical performance of seawater concrete was slightly lower than that of the freshwater-mixed concrete. The permeability performance of hardened concrete in the two mixtures was similar. Scanning electron microscopy and isothermal calorimetry were used as supplementary tools to better explain the experimental observations. Finally, remedial measures were proposed based on lab trials to improve the properties of seawater concrete.