Aidan Shields, CFO at Primark, has departed after six years in the role.
According to the Irish Independent, Shields has moved back to the United States with his family, where he was living prior to joining the value retailer.
His departure comes just as Primark is driving expansion in the US, having opened its first store there last September, in Boston.
A spokesman confirmed Shields' exit and said that a successor would be appointed in due course.
Prior to joining Primark, Shields had served at Gap and its Old Navy unit, as well as for KPMG in the United States.
In the financial year to the end of last September, Primark made a £673m profit. It now trades from 300 stores in 10 countries, having started as a standalone shop in Dublin in 1969.