As global temperatures rise, fish are predicted to become smaller. Body size is a fundamental trait that impacts many aspects of an animal's life history and ecology, and understanding how it may respond to climate change in particular fish groups, especially commercial or keystone species, is critical. The slimeheads (Family Trachichthyidae) include several commercially important species, but because they are deep‐dwelling, long‐lived fish that reproduce slowly, directly testing the temperature–size relationship in this family is challenging. Fortunately, Trachichthyidae have a long evolutionary history beginning in the Cretaceous, and their fossil record provides empirical data on the response of this family to past climate change events. In this study, we leveraged the extensive fossil record of the Late Cretaceous trachichthyid genus Hoplopteryx from the British Chalk Group of southern England, United Kingdom, to test whether its size declined at higher temperatures. Standard Lengths were measured from complete individuals and estimated from partial remains. Seawater palaeotemperature estimates were derived from oxygen stable isotope values (𝛿18O) of the bulk chalk rock surrounding the fossils using standard techniques and assumptions. Individual fish ranged from 56.3 to 262.6 mm in length, and measured seawater temperature estimates ranged from 19.5°C to 27.1°C. Multiple linear regression analyses revealed that the estimated seawater temperature was a significant negative predictor of Standard Length in the most common species, Hoplopteryx lewesiensis, supporting the prediction that higher temperatures led to smaller body size in fish. In addition, carbon stable isotope values (𝛿13C) also significantly negatively predicted the Standard Lengths of Hoplopteryx spp., suggesting that other environmental factors, such as primary productivity and/or the burial of organic matter, may also have affected body size.