Peptide natural products are a promising source of bioactive molecules. Antiproliferative activity on stimulated T-lymphocytes has been associated with plant-derived cyclic cystine-rich peptides from the Rubiaceae family. In this study, we report a correlation between the peptide diversity in eleven Psychotria extract samples, their prolyl oligopeptidase (POP) inhibitory activity, and antiproliferative effects on stimulated T-lymphocytes. The peptide fraction enriched with cyclotides from Psychotria solitudinum was primarily responsible for the antiproliferative activity, with low cytotoxicity compared to other extracts. We used the prototypic peptide psysol 2 to show that inhibition of POP was key, while inhibitors targeting prolyl oligopeptidases dipeptidylpeptidase-4 (DPP4) and fibroblast activation protein alpha (FAPα) did not yield the same phenotype. Interestingly, POP protein expression and enzymatic activity significantly increased in stimulated T-lymphocytes at 72 h post-activation, suggesting a role for POP in T-cell activation and proliferation. Unlike the prototype immunomodulatory cyclotide T20K, the POP inhibitors psysol 2 and KYP-2047 did not significantly affect NF-κB–dependent gene expression and pro-inflammatory cytokine expression on the mRNA level. However, POP inhibitors reduced caspase 3 and caspase 8 activation induced by T-cell receptor (TCR) and CD3 restimulation, providing partially protection of T-lymphocytes from AICD. Psysol 2 appears to be a new immunomodulatory cyclic cystine-rich peptide that functions via altered T-cell receptor–dependent caspase signaling in a POP-associated mechanism. Overall, this study provides new insights into the T-cell antiproliferative activity of natural cyclic peptides and their anti-inflammatory properties.