¹⁸F-interleukin-2 based PET imaging of activated T cells serves as a potential tool for non-invasive response prediction, treatment evaluation, and patient stratification in cancer immune checkpoint therapy. Herein, we report the radiolabelling of interleukin-2 (IL-2) with a novel arginine selective bioconjugation reagent, 4-[¹⁸F]fluorophenylglyoxal ([¹⁸F]FPG). Good non-decay corrected bioconjugation efficiencies of 29 ± 4 % (n = 5) were obtained for the [¹⁸F]FPG-IL-2. [¹⁸F]FPG-IL-2 uptake by the phytohemagglutinin-activated Jurkat cells (50.5 ± 1.2 %, n = 3) was significantly higher compared to the non-activated Jurkat cells (12.9 ± 1.1 %, n = 3). The [¹⁸F]FPG-IL-2 uptake was blocked by the pre-treatment of activated Jurkat cells with excess native IL-2 (22.3 ± 2.2 %, n = 3). Dynamic PET imaging and ex vivo biodistribution study of [¹⁸F]FPG-IL-2 in healthy and CT26 tumour bearing mice demonstrated hepatobiliary and renal clearance with minimal uptake in other organs and CT26 tumours. [¹⁸F]FPG-IL-2 PET imaging was applied to non-invasively monitor immune checkpoint therapy in CT26 tumour bearing mice, treated with IgG (control), ⍺PD-1 (monotherapy), and ⍺PD-1+⍺CTLA-4 (combination therapy). Significant uptake was observed in the spleens and tumours of the mice in the combination therapy group, which was associated with increased cytotoxic CD8+ T-cell infiltration and reduced tumour volumes. [¹⁸F]FPG-IL-2 based PET imaging has the potential to monitor immune checkpoint therapy.