The glucagon receptor (GCGR) is an emerging target in anti-diabetic therapy. Reliable biomarkers for in vivo activity on the GCGR, in the setting of dual glucagon-like peptide 1/glucagon (GLP-1/GCG) receptor agonism, are currently unavailable. Here, we investigated [68Ga]Ga-DO3A-S01-GCG as a biomarker for GCGR occupancy in liver, the tissue with highest GCGR expression, in non-human primates (NHP) by PET. [68Ga]Ga-DO3A-S01-GCG was evaluated by dynamic PET in NHPs by a dose escalation study design, where up to 67 µg/kg DO3A-S01-GCG peptide mass was co-injected. The test-retest reproducibility of [68Ga]Ga-DO3A-S01-GCG binding in liver was evaluated. Furthermore, we investigated the effect of pre-treatment with acylated glucagon agonist 1-GCG on [68Ga]Ga-DO3A-S01-GCG binding in liver. [68Ga]Ga-DO3A-S01-GCG bound to liver in vivo in a dose-dependent manner. Negligible peptide mass effect was observed for DO3A-S01-GCG doses <0.2 µg/kg. In vivo Kd for [68Ga]Ga-DO3A-S01-GCG corresponded to 0.7 µg/kg, which indicates high potency. The test-retest reproducibility for [68Ga]Ga-DO3A-S01-GCG binding in liver was 5.7 ± 7.9%. Pre-treatment with 1-GCG, an acylated glucagon agonist, resulted in a GCGR occupancy of 61.5 ± 9.1% in liver. Predicted human radiation dosimetry would allow for repeated annual [68Ga]Ga-DO3A-S01-GCG PET examinations. In summary, PET radioligand [68Ga]Ga-DO3A-S01-GCG is a quantitative biomarker of in vivo GCGR occupancy. © 2019, The Author(s).