BACKGROUND: Ceftriaxone (CTRX) and ampicillin/sulbactam (ABPC/SBT) are recommended by various guidelines as the first-line antibiotics for community-acquired pneumonia (CAP). However, which of these antibiotics is more effective for treating non-aspiration CAP remains unclear. METHODS: This study was a prospective, single-center, open-label, quasi-randomized controlled trial. Patients with adult CAP without risk for aspiration were allocated to either a CTRX or ABPC/SBT group based on the date of hospital admission. Macrolide was added to patients in each group. The primary outcome was the clinical response in the validated per-protocol (VPP) population at end of treatment (EOT). The secondary outcomes were clinical response during treatment and at end of study (EOS) in the VPP population, and mortality rate at day 30 in the modified intention-to-treat (MITT) population. RESULTS: Of 696 screened patients, 433 patients were excluded and 263 patients were allocated to receive either of the treatments. Males comprised 54% of patients and mean age and PSI were 62.1 ± 19.8 years and 69.3 ± 30.0, respectively, with 124 patients allocated to the CTRX group and 138 patients allocated to the ABPC/SBT group. The clinical effectiveness rate for the VPP population at EOT was 90% in the CTRX and 96% in the ABPC/SBT group (p = 0.072, 95% confidence interval [CI] of risk difference [RD]: - 12.6-0.8%). No significant difference in effectiveness at day 4 was observed between the CTRX and ABPC/SBT groups (p = 0.079, 95%CI of RD: - 12.1-0.4%), but at day 7, ABPC/SBT was significantly more effective than CTRX in the VPP population (p = 0.047, 95%CI of RD: - 13.3--0.4%). No significant difference in late response at EOS was seen between CTRX and ABPC/SBT groups: cure (89 [86%] and 102 [94%]), relapse (5 [5%] and 1 [1%]) and failure (10 [10%] and 5 [5%]; p = 0.053). Deaths within 30 days in MITT population was higher in CTRX group (4 [3%]) than in ABPC/SBT group (0 [0%]) (p = 0.048, 95%CI of RD: 0.1-6.3%). CONCLUSION: No significant difference in effectiveness was found between ABPC/SBT and CTRX at EOT. However, ABPC/SBT might be more effective in the early phase of treatment. TRIAL REGISTRATION: UMIN-CTR, UMIN000037464. Registered 25 July 2019 - Retrospectively registered, https://upload.umin.ac.jp/cgi-open-bin/ctr_e/ctr_view.cgi?recptno=R000042262.