Technologies for remote direct memory access (RDMA) congestion control include a requester device and a responder device in communication over an Ethernet network. The requester device sends routable RDMA packets to the responder device over the Ethernet network. The packets may be RDMA over Converged Ethernet version 2 (RoCEv2) packets. The responder device determines whether any of the received packets have been marked by the network with a congestion encountered codepoint. If so, the responder device sends an acknowledgment packet with an express congestion notification bit set in the RDMA base transport header. The requester device updates a congestion window as a function of a number of congested packets acknowledged and a total number of packets acknowledged. Those operations may be performed by a network controller of each of the requester device and the responder device. Other embodiments are described and claimed.