Various aspects of the present disclosure generally relate to wireless communication. In some aspects, a first user equipment (UE) may transmit, to a second UE, an initial transmission of a transport block via a sidelink unlicensed band. The UE may receive, from the second UE, a negative acknowledgement (NACK) based at least in part on the initial transmission of the transport block. The UE may transmit, to the second UE and based at least in part on the NACK and a retransmission reservation, a retransmission of the transport block via the sidelink unlicensed band in a retransmission occasion of a plurality of retransmission occasions associated with the retransmission reservation. Numerous other aspects are described.