This study reports highly permeable ultrathin film composite (uTFC) membranes whose rejection layer was reinforced by polymer chains during the interfacial polymerization of trimesoyl chloride (TMC) and m-phenylenediamine (MPD) to achieve enhanced salt rejection. A rejection layer of approximately 20 nm was formed at an MPD concentration of 0.01 wt%. This reinforced membrane had a water permeability of about 16.7 L/(m2 h bar), while exhibiting an improved divalent salt (Na2SO4) to monovalent salt (NaCl) selectivity compared with the control TFC membrane without reinforcement (3.44 vs. 1.06). The role of the reactable polymer chains in interfacial polymerization was discussed as MPD adsorbent and reactant, according to the measurements by quartz crystal microbalance and X-ray photoelectron spectroscopy. This work provides a new pathway for the design and construction of uniform ultrathin layers as well as the preparation of high performance separation membranes.