Polymer-cement composites produced by physical mixing are limited by their relatively low compressive strengths and weak interfacial bonding between two dissimilar phases. These limitations can be addressed by effectively inhibiting the aggregation of polymer chains. Herein, a series of cement/ polyacrylamide (PAM) composites is fabricated through the situ polymerization of acrylamide (AM) monomers during the early hydration stage of cement. In the designed composite, the polymer chemically interacts with cement hydration products and exhibits a randomly stretched distribution. The in situ polymerization of 1% well-dispersed monomers increases the flexural strength of the cement composite from 8.5 to 25.7 MPa, while maintaining its compressive strength. Compared with the polymer-cement paste obtained by adding 1% neat PAM, the designed composite attains a 53% and 26% improvement in flexural strength and compressive strength, respectively. This strategy is highly desirable for wide application of tough cementitious materials and provides insight into organic/inorganic composite designing.