Technical Abstract: A plant based thermal optimum approach to irrigation scheduling provides potential benefits to irrigation scheduling in that water applications are scheduled on the basis of plant water deficits. It is tempting for producers to employ this method, particularly in the wake of technologies, such as infrared thermometers, becoming increasingly more cost effective and reliable. Such irrigation systems require a defined thermal optimum for the crop and while such optimum values have been identified for cotton varieties in the USA, there is no information of this type for varieties common in Australian production. This paper outlines a three-fold approach to determining the optimum temperature (Topt) of the commercial Australian cotton cultivar Sicot 70BRF, in an Australian production system. It combines the use of a laboratory-based fluorescence assay, field-based carbon assimilation (A) and stomatal conductance (g) rates, and seasonal canopy temperature (Tc)-yield relations. The fluorescence assay showed a Topt between 28 and 30 °C, whilst leaf gas exchange rates showed a peaked at leaf temperature (Tl) of 29 °C. Seasonal Tc - yield relations peak at 26 °C, with yield reductions observed when Tc > 28 °C. We conclude the Topt of the Australian upland cotton cultivar Sicot 70BRF to be 28 ± 2 °C. This Topt will provide valuable information for use in thermal optimum irrigation scheduling systems.