Background: Molecular diagnostic testing is necessary to guide optimal first-line treatment. The number of patients who receive first-line treatment based on biomarker analysis in Japan is unknown. We aimed to determine the proportion of nonsquamous non-small cell lung cancer (NSCLC) patients for whom first-line treatment was selected based on biomarker testing. Methods: This retrospective, multicenter, observational study registered patients aged ⩾20 years with locally advanced or metastatic nonsquamous NSCLC who started first-line treatment between August and December 2017 in Japan. Data were collected from medical records between January and May 2018. The primary endpoint was the proportion of patients with confirmed biomarker status for first-line treatment decision. Results: Among 202 patients enrolled from 11 centers, 161 (79.7%; 95% confidence interval, 74.2–85.2%) had confirmed biomarker status. The testing rate was highest for epidermal growth factor receptor ( EGFR; 97.5%), followed by anaplastic lymphoma kinase ( ALK; 88.1%), programmed death ligand-1 (PD-L1; 87.1%), and ROS1 (67.3%). For first-line treatment, 70/75 patients with EGFR-positive tumors were administered an EGFR-TKI; 14/15 patients with ALK-positive tumors received an ALK inhibitor; 2/2 patients with ROS1-positive tumors received a ROS1 inhibitor; and 29/36 driver mutation-negative patients with a PD-L1 tumor proportion score ⩾50% were administered an anti-PD-1 monoclonal antibody. Median times from confirmed diagnosis date to first-line treatment initiation, and from first biomarker test order to last biomarker test result were 19 and 11 days, respectively. Conclusions: The proportion of nonsquamous NSCLC patients with confirmed biomarker status for first-line treatment was considered insufficient and in need of improvement.