Indirect-bandgap transition lasing, even under continuous-wave excitation at room temperature, is demonstrated in an ultra-thin WS2 disk. Small semiconductor lasers that can be integrated on a chip are essential for a wide range of optical applications, including optical computing, communication and sensing. Practical laser applications have only been developed with direct-bandgap materials because of a general belief that lasing action from indirect-bandgap materials is almost impossible. Here we report unexpected indirect-bandgap transition lasing in an ultra-thin WS2 disk. We demonstrate that a 50-nm-thick WS2 disk offers efficient optical gain and whispering gallery modes that are sufficient for lasing action. As a result, the WS2 disk exhibits indirect transition lasing, even under continuous-wave excitation at room temperature. Our experimental results are in close agreement with theoretical modelling for phonon-assisted photon lasing. The results derived from external cavity-free ultra-thin WS2 layers offer a new direction for van-der-Waals-material-based nanophotonics and introduce the possibility for optical devices based on indirect-bandgap materials.