In this paper, a cooperative scheme for secure device to device (D2D) communications underlaying cellular networks is proposed. In our scheme, the cellular base station (BS) wants to transmit its information to a cellular user (CU). Meanwhile, two devices want to communicate directly using the same spectrum used by cellular network with the help of some decode-and-forward (DF) relay nodes. In addition, there exists a malicious user which wants to eavesdrop on information transmission of D2D pair. The transmit power of the transmitter of the D2D pair (TD2D) and the relays is limited such that the outage performance of cellular network is satisfied. We study the performance of the proposed scheme, which is measured based on the outage probability, and obtain the closed form expression for the outage probability for the optimal relay selection scheme. Finally, the performance of the proposed scheme is evaluated using simulations.