Although semiconductor defects can often be detrimental to device performance, they are also responsible for the breadth of functionality exhibited by modern optoelectronic devices(1). Artificially engineered defects (so-called quantum dots) or naturally occurring defects in solids are currently being investigated for applications ranging from quantum information science(2,3) and optoelectronics(4) to high-resolution metrology(5). In parallel, the quantum confinement exhibited by atomically thin materials (semi-metals, semiconductors and insulators) has ushered in an era of flatland optoelectronics whose full potential is still being articulated(6-18). In this Letter we demonstrate the possibility of leveraging the atomically thin semiconductor tungsten diselenide (WSe2) as a host for quantum dot-like defects. We report that this previously unexplored solid-state quantum emitter in WSe2 generates single photons with emission properties that can be controlled via the application of external d.c. electric and magnetic fields. These new optically active quantum dots exhibit excited-state lifetimes on the order of 1 ns and remarkably large excitonic g-factors of 10. It is anticipated that WSe2 quantum dots will provide a novel platform for integrated solid-state quantum photonics(2,3) and quantum information processing(19), as well as a rich condensed-matter physics playground with which to explore the coupling of quantum dots and atomically thin semiconductors.