Halloran, who was formerly head of Asia-Pacific fixed income for UBS and went on gardening leave in July, will take up his new post at the end of September.
UBS confirmed Halloran's departure. Merrill declined to comment.
According to a source, the position is not related to debt capital markets, so Jon Pratt is still the head of DCM. Instead, Halloran will be responsible for secondary market fixed income (sales and trading) plus all currencies, commodities and principal investment work.
Halloran will continue to be based in Tokyo and will replace Antony Hung at Merrill, who has already been appointed head of Pacific Rim wealth management.