Highly sensitive and specific non-invasive molecular imaging methods are particularly desirable for the early detection of cancers. Here we report a near-infrared optical imaging probe highly specific to the hypoxic tumour microenvironment to detect tumour and cancer cells with the sensitivity to a few thousands cancer cells. This oxygen-sensitive, near-infrared emitting and water-soluble phosphorescent macromolecular probe can not only report the hypoxic tumour environment of various cancer models, including metastatic tumours in vivo, but can also detect a small amount of cancer cells before the formation of the tumour based on the increased oxygen consumption during cancer cell proliferation. Thus, the reported hypoxia-sensitive probe may offer an imaging tool for characterizing the tumour microenvironment in vivo, detecting cancer cells at a very early stage of tumour development and lymph node metastasis.