The principal goals of the present work are to measure the viscosity of vapor and liquid cis-1,1,1,4,4,4-hexafluoro-2-butene (R-1336mzz(Z)) and propose simplified correlations of the extracted data under saturation conditions for industrial design and simulation. R-1336mzz(Z), a hydro-fluoro-olefin with a low global warming potential and attractive environmental properties, can be considered as a potential working fluid for high-temperature heat pumps and organic Rankine cycles. Nevertheless, reliable experimental R-1336mzz(Z) viscosity data are lacking. This study presents R-1336mzz(Z) viscosity data measured by a tandem capillary tube method at temperatures from 314 (40.85 degrees C) to 434 K (160.85 degrees C) and 375 (101.85 degrees C) to 475 K (201.85 degrees C) for liquid and vapor phases, respectively, at pressures up to 4.06 MPa. Total standard uncertainties in liquid and gas viscosity measurements were lower than 3.04 and 3.21%, respectively.