Twelve species of edible seaweed from the coast of Korea were screened for skin moisturizing activity. We placed the lead of a Corneometer on an approximately 6-cm2 test area of the forearm and measured both untreated skin (control) and skin treated with test moisturizing creams either containing or not containing 5% water:propylene glycol (50:50) extracts of seaweeds. Over the 8-h observation period, the strongest activity of the Laminaria japonica extracts occurred at the 2-h period. For the 10% extract, hydration with the L. japonica extract increased by 14.44% compared with a placebo. Transepidermal water loss (TEWL) was also measured using a test cream with 10% L. japonica extract. For up to 8 h after applying the creams, TEWL was decreased to 4.01 g/cm2, which was approximately 20% of that seen with the control. We suggest that the L. japonica extract hydrates skin via the humectants and hydrocolloids that it contains. To confirm the safety of L. japonica extracts, we performed a patch test on human skin. The results suggested that at moderate doses humans can safely use the extracts. For commercial applications, we evaluated the physicochemical characteristics of the test cream products, including Hunter L, a, and b values; pH; refractive index; and coefficient of viscosity. L. japonica extract did not affect overall formulations of the test cream product in any of the tested aspects. These results suggest that L. japonica extract is a promising ingredient in moisturizing formulations.