Current and future injector designs for diesel engines approach pressures of greater than 100 MPa. However, the high-pressure physical properties, such as viscosity, of biodiesel blends with diesel fuel are nearly absent in the literature. This study focuses on the viscosity of biodiesel, diesel, and several biodiesel blends from 283.15 to 373.15 K and pressures up to 131 MPa. Soybean biodiesel (B100) and ultra-low-sulfur diesel (ULSD, B0) were combined to form biodiesel/diesel blends: B5, B10, B20, B40, B60, and B80. The viscosity of the samples increases linearly with the pressure until approximately 35 MPa, followed by a higher order response to pressure. The biodiesel blends with low biodiesel content (B5, B10, and B20) had very similar viscosities to ULSD. The normalized viscosity increased with a decreasing temperature and a decreasing biodiesel blend fraction. The viscosity of the various blends increased above the ambient pressure viscosity ranging from 164% at 373.15 K to 547% at 283.15 K at the highest pressure of 131 MPa. The biodiesel and some biodiesel blend samples at 283.15 K experienced pressure-induced cloud points (solid–liquid equilibrium) from 70 to 131 MPa. The Tait–Litovitz equation was found to correlate the biodiesel and diesel data with an absolute average relative deviation (AARD) of less than 1% over the large range of both temperature and pressure, and Kay’s mixing rule predicted the viscosity of the biodiesel blends with an AARD of 1.75%.