${\text{MoS}}_{2}$, a member of transition metal dichalcogenides (TMDs), has recently emerged as an interesting two-dimensional material due to its unique mechanical, thermal, electronic and optical properties. Unlike graphene which possesses massless Dirac fermions with ultrahigh electron mobility, monolayer ${\text{MoS}}_{2}$ is a direct band gap semiconductor. An interesting question arises: Can monolayer ${\text{MoS}}_{2}$ also possess massless Dirac fermions with ultrahigh electron mobility? Here, using first-principles calculations, we show that a monolayer ${\text{MoS}}_{2}$ allotrope, which consists of repeated square-octagon rings (abbreviated as so-${\text{MoS}}_{2}$ to distinguish it from the normal hexagonal lattice, h-${\text{MoS}}_{2}$) possesses both massless Dirac fermions and heavy fermions. Distinct from the $p$-orbital Dirac fermions of graphene, the Dirac fermions of so-${\text{MoS}}_{2}$ are $d$ electrons and possess a Fermi velocity comparable to that of graphene. The Dirac cone structure in so-${\text{MoS}}_{2}$ demonstrated here greatly enriches our understanding on the physical properties of TMDs and opens up possibilities for developing high-performance electronic or spintronic devices.