A method for manufacturing a composite part including a fibrous reinforcement and a thermoplastic polymer matrix, which includes the steps of i) depositing the fibrous reinforcement in a two-part mold having a seal compressible by evacuating said mold having no vent, ii) filling the evacuated mold by injecting, in the molten state, a two-component reactive system made of a reactive thermoplastic prepolymer, the injection being carried out using a device having two distinct compartments for each one of the two reactive components mixed in a static mixer, followed by a compression and compaction of the content of the mold, and iii) removing the part from the mold, without any finishing treatment. Further, a device for implementing said method, to the resulting part and to the use of said method in an “in-mold” assembly method.