In-line viscosity measurement systems and related methods may be useful in measuring the viscosity of a fluid in a flow path and, more specifically, in-line measuring the viscosity of a drilling fluid when integrated with drilling systems. For example, a method may include drilling a wellbore penetrating a subterranean formation while circulating a drilling fluid through the wellbore; measuring the viscosity of the drilling fluid with an in-line viscometer system after the drilling fluid has circulated through the wellbore, the in-line viscometer systems comprising either: (1) a two coaxial cylinder configuration, (2) a parallel plates configuration, or (3) a combination thereof positioned to allow for the drilling fluid to flow between the coaxial cylinders or parallel plates.