The protocol used for secure data transmission over the internet is HTTPS (Hypertext Transfer Protocol Secure). HTTPS encrypts data exchanged between a web browser and server, providing confidentiality and integrity during transmission. It uses SSL (Secure Sockets Layer) or TLS (Transport Layer Security) to establish a secure connection, making it essential for online transactions and protecting sensitive information from eavesdropping or tampering. Its widespread adoption enhances overall internet security.