Configuring service load balancers with specified backend virtual networks

Techniques are described for specifying a backend virtual network for a service load balancer. An example orchestrator of this disclosure is configured to receive a service definition for a service implemented by load balancing service traffic for the service among a plurality of backend virtual execution elements, wherein the service definition specifies a first virtual network to use as a backend virtual network for the service, to instantiate, in a selected one of the computing devices, a backend virtual execution element for the service, and to configure, based on the service definition specifying the first virtual network to use as the backend virtual network for the service, a network controller for the virtualized computing infrastructure to configure a load balancer to load balance service traffic to a first virtual network interface, of the backend virtual element, for the first virtual network.