  Van der Waals heterostructures made of graphene and transition metal
dichalcogenides (TMD) are an emerging platform for opto-electronic, -spintronic
and -valleytronic devices that could benefit from (i) strong light-matter
interactions and spin-valley locking in TMDs and (ii) exceptional electron and
spin transport in graphene. The operation of such devices requires significant
valley polarization and valley coherence, ideally up to room temperature. Here,
using a comprehensive Mueller polarimetry analysis, we report
\textit{artifact-free} room temperature degrees of valley polarization up to
$40~\%$ and, remarkably, of valley coherence up to $20~\%$ in monolayer
tungsten disulfide (WS$_2$)/graphene heterostructures. Valley contrasts have
been particularly elusive in molybdenum diselenide (MoSe$_2$), even at
cryogenic temperatures. Upon interfacing monolayer MoSe$_2$ with graphene, the
room temperature degrees of valley polarization and coherence are as high as
$14~\%$ and $20~\%$, respectively. Our results are discussed in light of recent
reports of highly efficient interlayer coupling and exciton transfer in
TMD/graphene heterostructures and hold promise for room temperature chiral
light-matter interactions and coherent opto-valleytronic devices.
