Mobile devices today provide a hardware-protected mode called Trusted Execution Environment (TEE) to help protect users from a compromised OS and hypervisor. Today TEE can only be leveraged either by vendor apps or by developers who work with the vendor. Since vendors consider third-party app code untrusted inside the TEE, to allow an app to leverage TEE, app developers have to write the app code in a tailored way to work with the vendor’s SDK. We proposed a novel design to integrate TEE with mobile OS to allow any app to leverage the TEE. Our design incorporates TEE support at the OS level, allowing apps to leverage the TEE without adding app-specific code into the TEE, and while using existing interface to interact with the mobile OS. We implemented our design, called TruZ-Droid, by integrating TrustZone TEE with the Android OS. TruZ-Droid allows apps to leverage the TEE to protect the following: (i) user’s secret input and confirmation, and (ii) sending of user’s secrets to the authorized server. We built a prototype using the TrustZone-enabled HiKey board to evaluate our design. We demonstrated TruZ-Droid’s effectiveness by adding new security features to existing apps to protect user’s sensitive information and attest user’s confirmation. TruZ-Droid’s real-world use case evaluation shows that apps can leverage TrustZone while using existing OS APIs. Our usability study proves that users can correctly interact with TruZ-Droid to protect their security sensitive activities and data.