No, Istanbul is not part of the European Union.
Istanbul is a major city in Turkey, which is a transcontinental country that spans both Europe and Asia. While a small portion of Istanbul lies in Europe, the city itself is not part of the European Union. Turkey has been a candidate for EU membership since the 1980s, but it has yet to meet the necessary criteria for full membership. Political, economic, and social factors have all played a role in this prolonged process. As of now, Turkey has a customs union with the EU but remains outside of the Union itself.