Dynamic deformation behavior of a commercial Ti-6Al-4V alloy is measured between room temperature and beyond the β-transus temperature with high thermal resolution using a rapid-heating Kolsky bar technique. The high thermal resolution allows for a thorough investigation of the dynamic thermal softening behavior of this alloy including effects related to the transformation from the initial hcp α/bcc β dual phase structure to a full β structure for improved modeling of high temperature dynamic manufacturing processes such as high-speed machining. Data are obtained at an average strain rate of 1800 s−1 from room temperature to 1177 °C, with total heating times limited to 3.5 s for all tests. Short heating times prevent thermal distortion of the Kolsky bar loading waves and can allow an investigation of non-equilibrium mechanical behavior, although no such behavior was identified in this study. Between 800 °C and 1000 °C, a progressive change in the thermal softening rate was observed that corresponded well with the equilibrium phase diagram for this alloy. The dynamic thermal softening behavior in the transformation region is incorporated via a new modification of the Johnson–Cook (J–C) viscoplastic constitutive equation. Rate sensitivity is determined at room temperature by combining Kolsky bar data with quasi-static measurements at strain rates from 7.5 × 10−5 s−1 to 0.16 s−1 and the data are fit using multi-parameter optimization to arrive at a full modified J–C model for Ti-6Al-4V to nearly 1200 °C. In its generic form, the modification factor we propose, G(T), is applicable to any material system undergoing gradual phase transformation over a range of temperatures.