Mobile ad-hoc networks (MANETs) consist of a set of communicating wireless mobile nodes or devices that could be deployed without the need for pre-established infrastructure for communication. Due to the insecure wireless communication medium and dynamic behavior of the nodes in MANETs, routing protocols are vulnerable to various security attacks, such as distributed denial of service (DDOS) attacks. DDOS attacks are used to temporarily disable network services by overloading the target system with huge traffic, such that it cannot respond to legitimate traffic. In this paper, we evaluate the performance of the Ad hoc on demand vector (AODV), Temporally ordered routing algorithm (TORA), Geographic routing protocol (GRP), and optimized link state routing (OLSR) routing protocols in MANETs under the DDOS attacks. These routing protocols are simulated using OPNET simulator to compare their performance using specific performance metrics on the network. The experimental results show that TORA protocol performs better than the AODV, OLSR, and GRP protocols under the DDOS attack.