This paper presents a finite element thermal model for linear friction welding applied to an instrumented weld in Ti6Al4V. The power at the weld interface was estimated from the measured transverse velocity and the cyclic machine load. This was compared with the power history reverse-engineered from thermocouple data. A simple analytical model captured the lateral distribution of heat input at the interface, while geometry changes and heat loss due to the expulsion of flash were included using a sequential step-wise technique, removing interface elements one layer at a time at discrete intervals. Comparison of predicted and experimental power showed a 20% discrepancy, attributed to uncertainty in the power estimate from force and displacement data, and sensitivity to the precision of locating the thermocouples. The thermal model is computationally efficient, and is sufficiently accurate for application to a new thermomechanical modelling approach, developed in a subsequent paper.