Securely joining devices to a mesh network using Datagram Transport Layer Security (DTLS) is described. A secure commissioning session is established between a joiner device and a commissioning device of the mesh network, in response to a joiner router receiving a DTLS-ClientHello message from the joining device, which is requesting to join the mesh network, and relaying the received message, encapsulated in a DTLS Relay Receive Notification message, to the commissioning device. The commissioning device transmits commissioning messages to the joining device over the secure session to enable the joining device to join the mesh network. The joiner router receives an indication from the commissioning device, that the joining device is to be entrusted to receive network credentials for the mesh network, and receives a key to securely transmit the network credentials from the joiner router to the joining device to complete commissioning the joining device to the mesh network.