Background: Dynamic tumor-tracking (DTT) stereotactic body radiotherapy (SBRT) with real-time monitoring for liver tumors using a gimbal-mounted linac can decrease the normal liver dose and planning target volume (PTV) without sacrificing the target dose. We hypothesized that this method would yield excellent tumor control comparable to that by the conventional SBRT. We conducted a multi-institutional prospective phase II study to evaluate the safety and efficacy of DTT-SBRT for liver tumors (UMIN-CTR, UMIN000017886). Methods: The main eligibility criteria were as follows: (1) < 4 liver tumors; (2) technical difficulties with percutaneous ablation therapies, inoperable lesions, or patient refusal to undergo surgery; (3) Eastern Cooperative Oncology Group performance status of 0–2; (4) Child-Pugh score of ≤8; and (5) respiratory motion of ≥10 mm. The primary endpoint was the local control rate (LC) at 2 years (expected endpoint: > 90%), and the secondary endpoints were overall survival rate (OS), progression-free survival rate (PFS), and treatment-related toxicity. We performed DTT-SBRT with 40 Gy in 5 fractions prescribed to the PTV D95, and a maximum dose of 55−58 Gy using 7–9 static non-coplanar ports of the 6-MV beam was applied. The Kaplan-Meier analysis was performed to estimate the OS and LC. Treatment-related toxicity was scored according to the Common Terminology Criteria for Adverse Events v.4.0. Results: Forty-eight patients (48 tumors) were registered from four institutes and successfully treated between September 2015 and March 2019. Their median age was 74 years (range, 52–97). Thirty-nine tumors were hepatocellular carcinomas, and nine were metastatic liver tumors. The median tumor size was 5.8 ml (range, 0.7–64.1). At the time of data cutoff on March 31, 2021, the median follow-up period was 36.5 months (range, 3.0–62.4). The 2-year LC was 98.0%. Two-year OS and PFS were 88.8% and 55.1%, respectively. Recurrences were observed in 33 patients (68.5%). One patient (0.2%) had local recurrence, 30 (62.5%) developed new hepatic lesions outside of the irradiation field, and 9 (18.8%) had distant metastases. Grade 3 adverse events were observed in seven patients, including elevation of liver or biliary enzyme levels and hematopoietic adverse events. No treatment-related toxicity of grade 4 or higher was observed. Conclusions: Employing DTT-SBRT to treat liver tumors resulted in excellent LC with acceptable incidence of adverse events. This method will be a promising alternative therapy for patients who cannot receive surgery or percutaneous ablation therapy. To the best of our knowledge, this is the first phase II trial evaluating DTT-SBRT for liver tumors. Clinical trial information: UMIN000017886.