The new has boosted hopes that other senior commanders could soon abandon their 18-year insurgency.
Mediator Betty Bigombe told Reuters in Kampala that Brigadier Sam Kolo gave himself up on Wednesday.
Kolo was the LRA's top negotiator in the group's first face-to-face peace talks with the government in a decade in December.
"Kolo came out today. We went on a mission to meet him and we have been successful," Bigombe told Reuters by telephone from the northern town of Gulu.
Colonel Charles Otema-Awany, head of military intelligence in northern Uganda, confirmed that Kolo had given himself up.
"It is true. Kolo surrendered today and we are very happy," he said, adding that Kolo, who had been the LRA's spokesman, was expected to arrive at the army's Fourth Division headquarters in Gulu later on Wednesday.