Low-grade fetal lung adenocarcinoma (L-FLAC) is an exceptionally rare pulmonary tumor, presenting with unclear histological and molecular features. In particular, the potential driver genes of L-FLAC remain elusive. To date, only five reports have documented genetic aberrations in L-FLAC. In the current study, we describe an unusual case of L-FLAC coexisting with adenocarcinoma in situ (AIS) of the lung, harboring different genetic mutations. A 39-year-old non-smoker female patient was referred to our hospital with the chief complaint of dyspnea for 20 days. Chest computed tomography (CT) revealed a 2.5×1.5×1.5 cm nodule in the right middle lobe, with no mediastinal lymph node enlargement or distant metastases. Thoracoscopic surgery was performed to remove the nodules. Histopathological analysis of the tissue sections, based on findings from immunohistochemical staining, confirmed a diagnosis of L-FLAC coexisting with AIS of the lung. Next-generation sequencing revealed L-FLAC-based mutations in DICER1 and CTNNB1, and AIS harboring KRAS mutations. Currently, the patient remains recurrence-free 17 months after the initial diagnosis. This report presents the first case demonstrating the coexistence of L-FLAC and AIS in the same pulmonary nodule, harboring different genetic mutations. Based on the literature review, this is the second reported case of L-FLAC bearing DICER1 mutations.