Electrocardiogram (ECG)-gated coronary CT angiography (CCTA) is a standard method for evaluating coronary artery disease. However, it has limitations, including patient discomfort, technical setup, and reduced accuracy in cases of arrhythmia or obesity. New CT technologies enable high-quality image acquisition without the need for ECG leads. This study compares image quality between ECG-less and ECG-gated CCTA. We retrospectively analyzed two matched groups of patients (n = 43 each) who underwent either ECG-less or ECG-gated CCTA. ECG-less datasets were retrospectively categorized into subphases corresponding to one-quarter, one-half, three-quarters, and full cardiac cycle reconstructions. Image quality was evaluated by radiologists using both subjective and objective measures for both groups and for all reconstructed datasets. Subjective assessment was performed using a 6-point Likert scale to rate motion artifacts in the left anterior descending artery (LAD), left circumflex artery (LCX), and right coronary artery (RCA). Objective image quality was quantified by calculating the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) in the left main coronary artery (LMCA), LAD, LCX, RCA, and ascending aorta. Image quality was comparable between the two groups in most subphases. However, in the ¼-cycle subphase in the ECG-less group, Likert scores were significantly lower for the LCX (P = 0.027), and RCA (P = 0.041) compared to the control group. Regarding SNR and CNR, there were no significant differences across all vascular territories in either full-phase or subphase data (P > 0.05). Both subjective and objective image quality metrics were comparable between ECG-less and conventional ECG-gated CCTA, supporting the reliability of ECG-less acquisitions.