Photothermal catalysis harnesses light-matter interactions that merge photochemical and thermochemical processes, driving surface reactions through localized heating effects. This approach holds the potential to unlock novel chemical transformations, but distinguishing between excited-state and ground-state reactions remains a significant challenge, as they occur simultaneously and can have similar effects. To accurately assess the contributions of these photochemical and thermochemical processes, spatially resolved, multiscale temperature measurements under operando conditions are essential. This review critically examines various temperature measurement techniques in photothermal catalysis, underscoring their pivotal role in advancing both scientific understanding and the development of optically and chemically engineered photocatalysts, along with photoreactors. Such advancements are key to facilitating the efficient and cost-effective production of sustainable chemicals and fuels from earth-abundant feedstocks, contributing to the transition toward a more sustainable future.