Membrane-based liquid filtration systems are key process engineering platforms for the chemical industry, particularly for solvent intensive processes. While they are used in a large scale for desalination, in water treatment, and in the food industry, polymeric membranes are mostly fabricated from solutions in organic solvents with concerning toxicity. Herein, we report a green fabrication method using decanoic acid as an alternative green solvent. The low vapor pressure, relative to common organic solvents, reduces the toxicity and the harm to the environment. The decanoic acid was used to dissolve trimesoyl chloride and reacted with polyethylenimine in the aqueous solution via an interfacial polymer reaction to produce a thin film composite membrane. The resultant membrane had high permeances for water (∼52 L m–2 h–1 bar–1) and organic solvents (16 to 124 L m–2 h–1 bar–1) and selectivity in the nanofiltration range. Therefore, our method of membrane preparation can offer an excellent and green platform for molecular separations for the chemical and biochemistry industry.