Legen merupakan minuman tradisional hasil sadapan nira pohon siwalan yang berpotensi mengalami fermentasi alami sehingga menghasilkan etanol. Penelitian ini bertujuan untuk menganalisis kadar etanol dalam sampel legen yang diperoleh dari kawasan Surabaya menggunakan metode Gas Chromatography-Flame Ionization Detector (GC-FID), serta mengevaluasi kesesuaiannya dengan ketentuan halal berdasarkan fatwa Majelis Ulama Indonesia (MUI) Nomor 10 Tahun 2018. Sebanyak tujuh sampel legen dikumpulkan secara acak dari pedagang kaki lima dan dianalisis kandungan etanolnya, disertai pengamatan karakteristik organoleptik (warna, aroma, dan viskositas) untuk mendeteksi indikasi fermentasi. Hasil menunjukkan kadar etanol bervariasi antara 0,15% (3B) hingga 11,31% (4B). Sampel dengan kandungan alkohol tertinggi memiliki aroma asam yang kuat yang mengindikasikan proses fermentasi terus berlanjut serta ketersediaan gula sebagai substrat fermentasi. Tiga sampel (1B, 2A, dan 4B) melebihi ambang batas etanol halal ( 0,5%) sebagaimana ditetapkan MUI, sehingga dinyatakan tidak memenuhi kriteria halal. Penelitian ini menegaskan pentingnya pengawasan mutu pada produk minuman fermentasi tradisional untuk memastikan keamanan konsumsi dan kesesuaiannya dengan syariat Islam.Legen is a traditional beverage obtained from the sap of the lontar palm, which has the potential for natural fermentation, thereby producing ethanol. This research aims to analyze the ethanol content in legen samples obtained from the Surabaya area using the GC-FID method, and to evaluate its compliance with halal provisions based on the MUI fatwa Number 10 of 2018. Seven legen samples were randomly collected from street vendors, and their ethanol content was analyzed, accompanied by observations of organoleptic characteristics (color, aroma, and viscosity) to detect indications of fermentation. The results showed ethanol content varied from 0.15% (3B) to 11.31% (4B). Samples with the highest alcohol content had a strong sour aroma characteristic, indicating that the fermentation process continued as well as the availability of sugar as a fermentation substrate. Three samples (1B, 2A, and 4B) exceeded the halal ethanol threshold ( 0.5%) as stipulated by MUI, thus being declared non-halal. This research emphasizes the importance of quality control in traditional fermented beverage products to ensure their safety for consumption and compliance with Islamic law.