The medium- to long-term healing effect and infiltration of inflammatory cells, after transplantation of allogeneic tendon-derived stem cell (TDSC) to the rat patellar tendon window wound, were examined. Allogeneic patellar TDSCs derived from a green fluorescent protein rat were used. The outcome of tendon healing and the infiltration of inflammatory cells were examined by histology and immunohistochemistry up to week 16 postinjury. The fate of the transplanted cells was examined by ex vivo fluorescent imaging and immunohistochemistry. Our results showed that the transplantation of allogeneic TDSCs promoted tendon healing with no increased risk of ectopic chondro-ossification up to week 16. A low infiltration of T cells, ED1 macrophages, ED2 macrophages, and mast cells in the window wound was obtained. The transplanted TDSCs were found in the window wound at week 1 and 2, but were absent after week 4 postinjury. In conclusion, allogeneic TDSCs promoted tendon repair in the medium to long term and exhibited weak immunoreactions and anti-inflammatory effects in the hosts after transplantation in a rat model. There was no increased risk of ectopic chondro-ossification after TDSC transplantation. The decrease in the number of transplanted cells with time suggested that allogeneic TDSCs did not promote tendon repair through direct differentiation.