An implantable device and associated method for delivering a multi-site pacing therapy includes electrodes for sensing cardiac signals and delivering cardiac pacing pulses to a first pacing site along a heart chamber and a therapy delivery module for delivering cardiac pacing pulses to a patient's heart via the electrodes. A sensing module measures an activation time at multiple pacing electrode sites along the heart chamber in response to delivering pacing pulses at the first pacing site. A controller is configured to identify a second pacing site from the plurality of pacing electrode sites in response to the activation times measured during pacing at the first site.