In situ measurement of chlorophyll concentration, with an optical meter, in the green seaweed Ulva ohnoi is presented to estimate the absolute chlorophyll content. The MC-100 optical meter (Apogee Instruments) was used during the study; the optical meter gives chlorophyll content in relative units as Chlorophyll Content Index units (CCI). Absolute chlorophyll content, in µmol m-2, was determined by extraction with dimethylformamide, and the chlorophyll concentration was measured with a spectrophotometer. Equations to convert CCI from optical meter to absolute chlorophyll content were presented for total chlorophyll, chlorophyll a, and chlorophyll b. Highly linear relationships (r2 above 0.9, n = 130) were found for total chlorophyll, chlorophyll a, and chlorophyll b. In situ non-destructive estimation of U. ohnoi chlorophyll content using the MC-100 portable chlorophyll optical meter was considered convenient, fast, and accurate.