Identifying tumor cells can be challenging due to cancer's complex and heterogeneous nature. Here, an efficacious phosphorescent probe that can precisely highlight tumor cells has been created. By combining the ruthenium(II) complex with oligonucleotides, we have developed a nanosized functional ruthenium(II) complex (Ru@DNA) with dimensions ranging from 300 to 500 nm. Our research demonstrates that Ru@DNA can readily traverse biomembranes via ATP-dependent endocytosis without carriers. Notably, the nanosized ruthenium(II) complex exhibits rapid and selective accumulation within tumor cells, possibly attributed to the nanoparticles' enhanced permeation and retention (EPR) effect. Ru@DNA can also effectively discern and label the transplanted cancer cells in the zebrafish model. Moreover, Ru@DNA is efficiently absorbed into the intestine and further distributed in the pancreas. Our findings underscore the potential of Ru@DNA as a DNA-based nanodevice derived from a functional ruthenium(II) complex. This innovative nanodevice holds promise as an efficient phosphorescent probe for both in vitro and in vivo imaging of living tumor cells.