Citi is the fourth US bank to hand a cash sum to campaign group Britain Stronger in Europe, after Goldman Sachs, JP Morgan and Morgan Stanley all donated.
A Britain Stronger in Europe spokesperson said: "We are a political campaign that is lucky enough to be benefiting from a range of donors and contributors – large and small. We will be publishing all details of our donations in line with Electoral Commission requirements in due course."
"Some businesses who are concerned about this issue will rightly want to make a contribution.
"We think it’s important to listen to what large UK employers say: that they would be less likely to invest in the UK if we were outside the EU and that our economy would be worse off."
Britain Stronger in Europe is expected to be named as the official out campaigning group by the Electoral Commission.
Executives at Citi agreed to donate "significantly in excess of" £100,000, possibly as high as £250,000, Mark Kleiman first reported. If towards the £250,000 mark, the donation would mean a total financial contribution from US banks of £1.5m.
Goldman Sachs and JP Morgan are understood to have pledged £500,000 each, with Morgan Stanley donating near £250,000.
Official campaign spending rules mean that both the officially designated Leave and Remain campaigns are only entitled to spend £7m in the final 10 weeks before the poll.
That's been under scrutiny itself, after the government last week spent £9m on a pro-EU leaflet.