Timber-concrete composite girders are a modern type of structure. They are formed by composite connection of a timber and concrete girder. They are designed in such a way that the concrete part of the girder is placed in the compression zone and the timber part of the girder in the tension zone. Various types of shear connectors are used for establishing this connection: from mechanical ones (bolts, nails, perforated plates, parts of steel sections, etc.) to chemical ones (various types of glue). The possibility of active strengthening of such girders with prestressed reinforcement is investigated in this paper. One possibility of applying external prestressing force is presented. Some strengthened-girder test results are presented and compared with results for similar girders but without strengthening.