Ni-based composite coatings have been extensively adopted in the automotive, printing and aerospace fields. However, numerous significant engineering components necessitate protective coatings with increased wear resistance. In the present study, the modification of the diamond surface morphology for improving the wear resistance of Ni-diamond composite coatings was experimentally verified. A new type of rough-surfaced diamond was obtained by applying the thermochemical corrosion method, and a Ni-diamond composite coating (#2) and Ni-rough-surfaced diamond composite coating (#3) were prepared by means of direct current electrodeposition. The present research shows that rough-surfaced diamond has a larger surface area and higher electronegativity than ordinary diamond, which enhances the adhesion between the Ni-based bond and the rough-surfaced diamond. Compared with the #2 coating, the microhardness of the #3 coating was increased to 613 HV and the coefficient of friction was reduced to 0.63. Due to the synergistic influence of augmented microhardness and bond strength, the wear rate of the #3 composite coating experienced a decline of 15.6%, resulting in a reduction to 1.79 × 10−5 mm3/Nm. Such enhancement consequently led to an improvement in the wear resistance of the coating. The application of rough-surfaced diamonds offers a new direction for the development of new high wear-resistant coatings with broad application prospects.