Background: Visual assessment of coronary artery calcium (CAC) on ungated chest CT has been described previously. However, its reliability and clinical utility remain uncertain, particularly in PET/CT studies that use low-dose, low-slice CT and are susceptible to respiratory artifacts. Methods: We retrospectively analyzed 106 patients (median age, 66 years [interquartile range, 60–75 years]; 67 men [63.2%]) who underwent PET/CT and electrocardiogram (ECG)-gated chest CT within a 90-day interval. Six readers (three radiologists and three nuclear medicine physicians) independently assessed CAC on PET/CT using a standard four-point visual scale and a 0–12 ordinal scale based solely on written instructions. Agatston scoring was also performed. Interobserver agreement and concordance with ECG-gated chest CT Agatston score categories were calculated. Major adverse cardiovascular events (MACE) were recorded over a median follow-up of 3.5 years. Results: Interobserver agreement was good for both the standard visual (κ = 0.761) and ordinal (κ = 0.779) scales. Concordance with ECG-gated CT Agatston categories was higher for standard visual (κ = 0.849) and ordinal (κ = 0.750) scoring than for PET/CT Agatston categories (κ = 0.464). Both qualitative scales tended to underestimate CAC categories compared with ECG-gated CT; however, severe CAC on PET/CT predicted MACE (hazard ratios: 4.41 standard visual; 6.59 ordinal), and the ordinal scale significantly stratified MACE-free survival (p = 0.047). Conclusions: Standard visual and ordinal CAC scoring on the ungated CT portion of PET/CT is quick, reproducible, closely mirrors ECG-gated-CT Agatston grading, and offers prognostic value for future MACE in cancer patients.