DS-8201a in Human Epidermal Growth Factor Receptor 2 (HER2)-Expressing or -Mutated Non-Small Cell Lung Cancer

Inclusion Criteria: * Age ≥20 years old in Japan, ≥18 years old in other countries * Pathologically documented unresectable and/or metastatic non-squamous NSCLC * Has relapsed from or is refractory to standard treatment or for which no standard treatment is available * For Cohort 1 and Cohort 1a: HER2-overexpression (IHC 2+ or 3+) status must be assessed and confirmed by Clinical Laboratory Improvement Amendments (CLIA)-certified laboratory or equivalent, from an archival tumor tissue sample * For Cohort 2 only: Participant has any known documented activating HER2 mutation from an archival tumor tissue sample analyzed by CLIA laboratory or equivalent. Note: HER2 mutation documented only from a liquid biopsy sample cannot be used for enrollment. * Presence of at least 1 measurable lesion assessed by the investigator and based on Response Evaluation Criteria in Solid Tumors (RECIST) version 1.1 * Is willing and able to provide an adequate archival tumor tissue sample * Is willing to undergo a tissue biopsy, after the completion of the most recent treatment regimen * Has Eastern Cooperative Oncology Group Performance Status (ECOG PS) of 0 to 1 Exclusion Criteria: * Had been previously treated with HER2-targeted therapies, except for pan-HER class tyrosine kinase inhibitors * For Cohort 1 and Cohort 1a: Has known HER2 mutation * Has a medical history of myocardial infarction, symptomatic congestive heart failure (CHF) (NYHA classes II-IV), unstable angina or serious cardiac arrhythmia * Has a history of (non-infectious) interstitial lung disease (ILD)/pneumonitis that required steroids, or current ILD/pneumonitis, or suspected ILD/pneumonitis that cannot be ruled out due to imaging at screening * Has a QT interval corrected by Fridericia's formula (QTcF) prolongation to \> 450 millisecond (ms) in males and \> 470 ms in females * Has a medical history of clinically significant lung disease * Is suspected to have certain other protocol-defined diseases based on imaging at screening period * Has history of any disease, metastatic condition, drug/medication use or other condition that might, per protocol or in the opinion of the investigator, compromise: 1. safety or well-being of the participant or offspring 2. safety of study staff 3. analysis of results