  Energy transfer between fluorescent probes lies at the heart of many
applications ranging from bio-sensing and -imaging to enhanced photo-detection
and light harvesting. In this work, we study F\"orster resonance energy
transfer (FRET) between shallow defects in diamond --- nitrogen-vacancy (NV)
centers --- and atomically-thin, two-dimensional materials --- tungsten
diselenide (WSe$_2$). By means of fluorescence lifetime imaging, we demonstrate
the occurrence of FRET in the WSe$_2$/NV system. Further, we show that in the
coupled system, NV centers provide an additional excitation pathway for WSe$_2$
photoluminescence. Our results constitute the first step towards the
realization of hybrid quantum systems involving single-crystal diamond and
two-dimensional materials that may lead to new strategies for studying and
controlling spin transfer phenomena and spin valley physics.
