This paper presents a study on high temperature cyclic plasticity of a welded P91 T-joint under cyclic internal pressure, in the context of high temperature low cycle fatigue (HTLCF) performance of such connections. In the present work, attention is focused on the development of a multi-material model for high temperature cyclic plasticity, including the effects of the different weld-related material zones, namely, parent metal, weld metal and heat-affected zone. The cyclic plasticity behaviour of the three zones is identified from previously published high temperature, low cycle fatigue test results on uniaxial test specimens, including parent metal, weld metal and cross-weld specimens, obtained from a specially fabricated pipe girth weld, using ex-service P91 material. The cyclic plasticity material model includes the effects of kinematic hardening and cyclic softening. A three-dimensional finite element model of the welded T-joint is developed, incorporating the three sets of identified cyclic plasticity constants. The study is limited to isothermal conditions of 500 degrees C, with a view to understanding the complex effects of multiple material zones with inhomogeneous cyclic plasticity behaviour. The heat affected zone is shown to play a key role in the development of plastic strains and localised stresses. The particular T-joint geometry is the subject of an investigation due to premature failure in a combined cycle gas turbine plant.