A semi-analytical model for predicting diffuse reflectance of coastal and oceanic waters is developed based on the water-column optical properties and illumination conditions. Diffuse reflectance (R) is an apparent optical property that is related to the Gordon's parameter (bb/(a+bb)) through a proportionality factor "f". The conventional assumption of "f" as a constant (0.33) yields large errors in case of turbid and productive coastal waters and a predictive model based on this assumption is generally restricted to open-ocean waters (low chlorophyll case). In this paper, we have sorted the dependent factors that influence "f" values in the water column. Here, the parameter "f" is modeled as a function of wavelength, depth, inherent optical properties (IOPs) and illumination conditions. This work eliminates the spectral constants (KChl and KSS) associated with our previous model and constrains the present model to be solely dependent on the IOPs and illumination conditions. Data used for parameterization and validation are obtained from in situ measurements in different waters within coastal environments. Validation shows good agreement between the model R and in situ R values with the overall mean relative error of less than a few percent. The model is valid for a wide range waters within coastal and open-ocean environments. © Author(s) 2015.