Industrially, ultrafiltration (UF) membranes are produced through non-solvent induced phase separation (NIPS), but due to environmental hazards inherent to the NIPS process as well as the low surface porosity and fouling resistance of membranes produced via this method, an alternative route to UF membranes is desirable. This work presents the self-assembly of Pluronic block copolymers in the presence of water and a monomeric phase as a new technique for the preparation of UF membranes without the need for organic solvent or post-modification. Different compositions of block copolymer, water, and monomer were polymerized to obtain both hexagonal and lamellar mesostructures, as indicated by small angle X-ray scattering (SAXS) and cross-polarized light microscopy. As-synthesized membranes were found to have pore sizes in the range of 3-4 nm with a molecular weight cutoff of 1500 g/mol and displayed both excellent fouling resistance and high permeance of water, vastly outperforming a conventional NIPS UF membrane. Further, in contrast to NIPS, the proposed method provides flexibility in terms of both the final membrane chemistry and pore size. As such, it is a versatile approach that can be easily tailored to produce membranes for a wide range of applications including wastewater treatment and food processing.