Only mismatch repair (MMR)-deficient colorectal cancer (CRC) appears to respond well to programmed death (PD)-1 inhibition at the present time. Emerging evidence suggests a role for micro-environmental factors such as CD25⁺ cells modulating response to PD-1 inhibition. In the Apc^Min/+ model of familial adenomatous polyposis (MMR-proficient CRC), increased Cyclooxygenase-2 (Cox-2) expression by cells which include alternatively activated mononuclear phagocytes promotes intestinal tumorigenesis by mechanisms which may include immune suppression. To gain insight into this, we compared regulatory T cell (T_reg) populations between Apc^Min/+ and wild-type mice prior to and after the phase of increased intestinal Cox-2-dependent prostaglandin E₂ (PGE₂) production. There was no difference in systemic T_reg function or numbers between Apc^Min/+ and wild-type mice. However, increased numbers of small intestinal CD25⁺ T_regs were observed with increased Cox-2 activity in the absence of any difference in the expression of Tgf-β or Tslp between Apc^Min/+ and wild-type mice. Cox-2 inhibitor therapy (Celecoxib) reversed the increase in Apc^Min/+ intestinal CD25⁺ T_reg numbers, without decreasing numbers of CD25⁺ systemic T_regs. Forkhead box protein 3 (FoxP3⁺) and Cox-2⁺ cells were co-localized to the interstitium of adenomas of Apc^min/+ mice. These results suggest selective dependence of an ‘activated T_reg’ phenotype on paracrine Cox-2 activity in Apc^Min/+ small intestine. For therapeutic potential, further studies are required to evaluate the relevance of these findings to human cancer as well as the functional significance of CD25⁺ intestinal T_regs in cancer.