Yttrium-90, the daughter product of 90Sr decay, is separated and concentrated from up to 40L samples of seawater. A ferric hydroxide precipitate enhanced with 10mg of lanthanum and 1mg of yttrium concentrates 90Y, while rejecting much of the salt content of the seawater sample. A second precipitation with lanthanum fluoride removes additional matrix ions. Yttrium is separated from potentially interfering radionuclides in the sample, including rare earths such as 138La and 139/144Ce, using a 2mL cartridge of Eichrom DGA Resin. 90Y is measured on a low background gas flow proportional counter following cerium fluoride microprecipitation onto an Eichrom Resolve® Filter. Chemical yield of stable yttrium is determined by ICP-MS or ICP-AES. Average chemical recovery of yttrium is 84 + 7% for 40L samples. Measured values of 90Sr(90Y) agree to within 5% of reference values, with two hour count times. The minimum detectable activity for 90Sr for 40L samples with a two hour count time is 0.35mBq/L. The average time to complete the method is 8 hours. While standard methods targeting Sr are limited by the ~8mg/L native Sr content in seawater, targeting 90Y directly allows for the efficient processing of very large seawater samples to achieve very low minimum detectible activities. However, interference by the fission product 91Y (t1/2 = 58.51 days) precludes application of this method for the measurement of 90Sr(90Y) immediately following a radiological incident involving the release of un-aged nuclear fuel or fission products.