The Financial Conduct Authority said the bank had failed to manage conflicts of interest between itself and its customers in the way the price of gold was set, between 2004 and 2013.
The FCA focused on the behaviour of Daniel James Plunkett, a trader on the Barclays precious metals desk, on June 28, 2012.
The FCA said Mr Plunkett exploited a weaknesses in Barclays' systems and controls to influence the 3pm gold fixing. As a result, Barclays did not have to make a £2.3 million payment to a customer, although it later compensated the customer in full.
The FCA has since fined Mr Plunkett £95,600 and banned him from the industry.