Among the noble-metal clusters, very few reports about platinum clusters were used as bio-imaging probes of tumors except as a reducing catalyst. It is firstly established herein that the biocompatible platinum nanoclusters are spontaneously biosynthesized by cancerous cell (i.e., HepG2, human hepatocarcinoma; A549, lung cancer and others) rather than the non-cancerous cells (i.e., L02, human embryo liver cells) when incubated with micromolar chloroplatinic acid solutions. These in situ bio-synthesized platinum nanoclusters could be readily realized in biological environment and emit a bright fluorescence at 460 nm, which could be further utilized to facilitate an excellent cancer cell-killing efficiency when combining with porphyrin derivatives for photothermal treatment. This raises the possibility of providing a promising and precise bio-imaging strategy for specific fluorescent self-bio-marking of tumor locations and realize fluorescence imaging-guided photothermal therapy of tumors.