Determining the upper (CT_max) and lower (CT_min) critical thermal limits of invasive ants provides insight into how temperature could shape their distribution, seasonality, and daily activity. Understanding the potential distribution of invasive ants is imperative to improving quarantine and management efforts. Nylanderia fulva (Mayr) (tawny crazy ant) and Solenopsis invicta (Buren) (red imported fire ant) are invasive ants that are established throughout the southeastern United States. Recent studies have found that body size and thermal ramping rate can affect the estimation of critical thermal limits. However, the effects of both variables and their interactions on the thermal limits of N. fulva and S. invicta have not previously been described. Thus, we evaluated the impacts of body size and ramping rate on the critical thermal limits of N. fulva and S. invicta. Overall, N. fulva had a narrower thermal breadth than S. invicta (Nf CT_min = 7.3° C and Nf CT_max = 41.3° C vs. Si CT_min = 4.1° C and Si CT_max = 45.3° C). For both species, slower ramping rates resulted in lower CT_max values and ants with smaller head capsules had a narrower thermal breadth than ants with larger head capsules. These data improve our understanding of the critical thermal limits of both species and could be useful for developing predictive models that estimate the future spread of these invasive ants in nonnative ranges.