The capacity for thermal adjustment or adaptation is critical to population persistence in a warming ocean. Understanding such performance across a species’ range can give insights into the extent of possible adjustment as well as spatial variation in vulnerability. This study tested the effects of temperature on the density and size of gametophytes of the habitat-forming kelp Ecklonia radiata, across its Australian distribution (between ~9° and ~33°S). Gametophytes from warm, intermediate and cool biogeographic regions were cultured over a temperature gradient from 12 to 26°C, revealing optimum temperatures of ~18 to 23°C—well above current maximum temperatures in parts of the range—and a positive relationship between in situ temperature and thermal optima for performance. Optimum temperatures in warmer regions were more than 1°C higher than in cooler regions. Biogeographically, the thermal optima for gametophytes were more strongly linked to long-term temperature characteristics of a region (annual extremes, 3 yr range) than short-term variation (mean for the month prior) in sea temperature. These results document that present-day populations of E. radiata have adjusted their gametophyte thermal sensitivity according to their local environment, and further indicate that these differences are adaptive rather than phenotypic. Collectively, these findings suggest that the scope for thermal adaptation and gametophyte performance of E. radiata across most of its Australian distribution is within projected levels of future warming. ; Margaret B. Mohring, Thomas Wernberg, Jeffrey T. Wright, Sean D. Connell, Bayden D. Russell