Background: Although the number of older adult patients (aged ≥ 75 years) with pancreatic cancer (PC) is increasing, there are still limited data regarding chemotherapy for these patients, who experience adverse events more frequently than younger patients with PC. In this study, we evaluated the efficacy and safety of biweekly gemcitabine plus nab-paclitaxel (GnP) as first-line therapy for older adult patients with unresectable PC. Methods: We conducted a single-center prospective study. Patients aged ≥ 75 years with pathologically proven unresectable pancreatic adenocarcinoma were enrolled after obtaining written informed consent. Patients were treated with a modified regimen of gemcitabine (1000 mg/m²) and nab-paclitaxel (125 mg/m²) on days 1, 15 of every 28-day cycle. The primary endpoint was the overall response rate. Secondary endpoints included disease control rate, overall survival, progression-free survival, 1-year survival rate, 2-year survival rate, and adverse events. Results: From August 2018 to March 2021, 17 patients were enrolled (locally advanced 4; metastatic 13). Five patients (29.4%) were men, and the median age was 77 years (range: 75-81). The median Geriatric 8 score was 13 points (range 6-17). Biliary drainage was performed in three patients. The overall response rate was 47.1% (8/17). Disease control rate was 82.4% (14/17). Median overall survival was 16.8 months (95% confidence interval (CI) 5.9-24.6) and median progression-free survival was 8.3 months (95% CI: 4.9-10.1), respectively. The 1-year survival rate and 2-year survival rates were 58.8% and 29.4%, respectively. Grade 3 or 4 hematologic adverse events and non-hematologic adverse events were observed in 17.6% (3/17) and 29.4% (5/17), respectively. No deaths was observed in association with biweekly GnP regimen. Conclusions: Biweekly GnP as first-line therapy can provide favorable treatment outcomes with tolerable toxicity in older adult patients with unresectable PC. Clinical trial information: jRCTs051190038.