In recent years, there has been considerable interest in Intercloud. Inspired by the Internet, Intercloud allows clouds to be interconnected, forming a cloud/network of clouds. With Intercloud, many new and powerful cloud computing services can be provided. One of the basic problems with supporting Intercloud is the need to address the issue of how to ensure that clouds can communicate with each other effectively and efficiently. Inspired by the HyperText Transfer Protocol (HTTP), this paper presents an Intercloud Communications Protocol (ICCP). ICCP allows clouds to communicate with each other using a common protocol, which is transparent to the underlying application programming interfaces of different clouds. It also includes security functions to ensure confidentiality and integrity. Essentially, clouds can communicate with each other based on XML-based request/response messages in a secure manner. An Intercloud Gateway prototype has been developed to demonstrate the basic functionalities of the ICCP, such as transferring data objects securely. The proposed framework is expected to inspire innovative cloud computing services as well as pose challenging research problems.