Abstract: There are provided systems and methods for iframe injection in mobile web browser applications for web browser extension opt-in. A service provider may provide a mobile application web browser extension, which may operate in conjunction with a mobile application web browser. The extension may interface with the web browser in order to determine data for browsed websites and user interactions and provide offers and savings to users during electronic transaction processing. In order to use the extension with the web browser, an opt-in preference and permission may be required. To provide this opt-in, the extension may cause the web browser to navigate to and load a webpage of the service provider. The extension may then inject an iframe that calls another domain, and a script of the extension executes in the iframe. The script may then infer that a permission has been granted.