The shrinkage behavior of cementitious materials mixed with seawater is investigated. Cement mortar mixtures were prepared with two water-to-cementitious materials ratios (w/cm = 0.36 and 0.45), two binder compositions (namely, ordinary portland cement (OPC) and OPC with 20 % fly ash replacement), and two types of water (tap water and seawater). The autogenous and drying shrinkage behavior of these mixtures are examined using ASTM standard test methods for 65 days. The use of seawater as mixing water increased the autogenous shrinkage. At w/cm 0.36, the ultimate autogenous shrinkage increased from 213 μs in the mixture with tap water to 387 μs in the mixture with seawater; the corresponding values were 149 and 314 μs, respectively, for mixtures with w/cm 0.45. An acceleration of the cement hydration at early ages caused by the seawater is identified as the cause of the increase in autogenous shrinkage in mixtures with seawater. At w/cm 0.36, seawater did not have a strong effect on the drying shrinkage, and tested mixtures had ultimate drying shrinkage values between 543 and 663 μs. At w/cm 0.45, in mixtures without fly ash, ultimate drying shrinkage increased from 838 μs in the mixture with tap water to 1,027 μs in the mixture with seawater. In mixtures with fly ash, the ultimate drying shrinkage increased from 738 μs in the mixture with tap water to 1,370 μs in the mixture with seawater. The drastic increase in the drying shrinkage in mixtures containing fly ash and seawater at w/cm 0.45 seems to be due to changes in mass loss behavior and the development of a finer pore size distribution. In applications where drying shrinkage may be a concern, the use of fly ash in seawater-mixed concrete could be problematic and lead to increased cracking risk. While the trends observed here will also hold in concrete, quantifying drying shrinkage and cracking of concrete based on the drying shrinkage of mortar mixtures is complex and depends on many other factors.