Inclusion Criteria:  Must have histologically confirmed Stage IB, II, or IIIA NSCLC. Must have an activating RET gene fusion in tumor based on polymerase chain reaction (PCR) or next generation sequencing (NGS).  Must have received definitive locoregional therapy with curative intent (surgery or radiotherapy) for Stage IB, II, or IIIA NSCLC.  -- Must have undergone the available anti-cancer therapy (including chemotherapy or durvalumab) or not be suitable for it, based on the investigator's discretion.  Must have completely recovered from definitive therapy (surgery or radiotherapy) as well as adjuvant therapy at the time of randomization.  Maximum time allowed between definitive therapy completion and randomization must be:  10 weeks if no chemotherapy was administered 26 weeks if adjuvant chemotherapy was administered Eastern Cooperative Oncology Group (ECOG) performance status of 0-1. Adequate hematologic, hepatic, and renal function. Willingness of men and women of reproductive potential to observe conventional and highly effective birth control for the duration of the study and for at least 2 weeks after last dose of study drug.  Exclusion Criteria:  Additional oncogenic drivers in NSCLC, if known. Evidence of small cell lung cancer. Clinical or radiologic evidence of disease recurrence or progression following definitive therapy. Known or suspected interstitial fibrosis or interstitial lung disease or history of (noninfectious) pneumonitis that required steroids. Clinically significant active cardiovascular disease or history of myocardial infarction within six months prior to planned start of selpercatinib or prolongation of the QT interval corrected for heart rate using Fridericia's formula (QTcF) greater than 470 milliseconds. Have known uncontrolled human immunodeficiency virus (HIV)-1/2 infection. Have known active hepatitis B or C. Active uncontrolled systemic bacterial, viral, or fungal infection or serious ongoing intercurrent illness, such as hypertension or diabetes, despite optimal treatment. Major surgery within 4 weeks prior to planned start of selpercatinib. Clinically significant active malabsorption syndrome or other condition likely to affect gastrointestinal absorption of the study drug. Other malignancy unless nonmelanoma skin cancer, carcinoma in situ of the cervix or other in situ cancers or a malignancy diagnosed greater than or equal to two years previously and not currently active. Pregnancy or lactation. Prior treatment with a selective RET inhibitor (e.g. selpercatinib or pralsetinib).