Instant messaging is a form of online, real time form of communication between two or more people based on typed text. IM system has grown rapidly among network users. But most of existing instant messaging systems have severe security problems, people want to retain their privacy and communication should not copy or modified by a third party. This thesis presents the design and implementation of a secure instant messaging system. It achieved security objectives such as data integrity and confidentiality through encryption. It ensures that the conversation is only read by intended recipient. The name of the proposed system is chosen to be SIMSM (the acronym for SIMple Secure Messenger). SIMSM is designed for local networks. Based on client-server connection, it enables users to send and receive secure instant messaging between them. No internet connection is required. Easily in sending and receiving text messages. It supports standard messaging features such as private chat, group chat (conference), message notification and encryption. The primary constituting modules are: Registration Module which identifies users to the system; Login Module which allows users to access the system; Sign-In Problem Module that deals with forgetting the identification (ID) and password problem; Private Chat Module that enable private chat between online users; and Conference Chat Module that enable more than one online user to chat with each others. The proposed IM system has been evaluated according to two important factors in instant messaging: security and time consuming. Many test cases were taken to show that SIMSM is quite suitable for secure chatting service. The proposed secure instant messaging had been established using windows API functions with Java platform version 6.0.0.105 programming language.