A connectionless service is a concept in data communications used to transfer data at the transport layer (layer 4) of the OSI model. It describes the communication between two nodes or terminals in which data is sent from one node to the other without first ensuring that the destination is available and ready to receive the data. A session connection between the sender and the receiver is not required, the sender just starts sending the data. The message or datagram is sent without prior arrangement, which is less reliable but faster transaction than a connection-oriented service.

User Datagram Protocol (UDP) is a connectionless protocol, while Transmission Control Protocol (TCP) is a connection-oriented network protocol.