Near simultaneously (within 2 hours) acquired ERS-2 Synthetic Aperture Radar (SAR) and Sea-viewing Wide Field-of-view Scanner (SeaWiFS) data over an upwelling region shows remarkable similarity in feature's location, scale, and boundary. Under uniform wind and sea states, reduction of Normalised Radar Cross Section (NRCS) from SAR is found highly correlated with the increase of SeaWiFS Chlorophyll-a (Chl-a) concentration. Typically, 1 mg/m3 of chlorophyll-a is correspondent with a 5 dB reduction of NRCS. This relationship is supported by simultaneously collected in situ sea truth measurements. This work provides a direct evidence and exceptional quantification of the relationship between radar backscatter and ocean colour that has long been speculated.