Singlet oxygen (1O2) is an electronic state of molecular oxygen which plays a major role in many chemical and biological photo-oxidation processes. It has a high chemical reactivity which is commonly harnessed for therapeutic issues. Indeed, 1O2 is believed to be the major cytotoxic agent in photodynamic therapy. In this treatment of cancer, 1O2 is created, among other reactive species, by an indirect transfer of energy from light to molecular oxygen via excitation of a photosensitizer (PS). This PS is believed to be necessary to obtain an efficient 1O2 production. In this paper, we demonstrate that production of 1O2 is achieved in living cells from PS-free 1270 nm laser excitation of molecular oxygen. The quantity of 1O2 produced in this way is sufficient to induce an oxidative stress leading to cell death. Other effects such as thermal stress are discriminated and we conclude that cell death is only due to 1O2 creation. This new simplified scheme of 1O2 activation can be seen as a breakthrough for phototherapies of malignant diseases and/or as a noninvasive possibility to generate reactive oxygen species in a tightly controlled manner.