We used a 2.5-dimensional thermomechanical ice-sheet model including the oxygen-isotope ratio 18O/16O as a passive tracer to simulate the isotopic composition (δ18O) of the North American Ice Sheet (NAIS) during the last glacial cycle. This model allowed us to estimate the NAIS contribution to the change of seawater δ18O (δw) between the Last Glacial Maximum (LGM) and the Holocene and to evaluate the effect of nonequilibrium isotopic composition of the NAIS on the relationship between ice-volume variations and the ocean isotopic enrichment. The enrichment due to the NAIS at the LGM was 0.63?, corresponding to ~60% of the LGM sea-level lowstand and to a mean δ18O of the NAIS of approximately -31?. The modeled NAIS volume variations and the induced δw changes over the past 120,000 years indicated no significant time lag. The inaccuracy associated with linearly inferring ice-volume variations from δw changes was generally less than 10%.