Kieron McKeefery, 21, said he "walked around like the king for a week" after finding £1,245,000 in his NatWest account.
The web designer, from Barnsley, discovered the cash when he checked his unused savings account at work.
NatWest said it was not their error but the funds had been sent to Mr McKeefery's account by another bank.
Mr McKeefery, from the Brampton area, alerted NatWest straight away after the find in January saying that having so much money made him feel "uneasy".
He has just released the details, and said: "My first thought was 'get it spent', but about five seconds later I thought I should actually tell the bank."
He said he was surprised by the laid-back reaction of NatWest, saying it took 10 days to take the money back.
"I constantly had to chase them up to get it sorted - I didn't really want to leave such a large amount in there."
NatWest admitted it should have done more to move the money from Mr McKeefery's account to a suspense account.
"We appreciate Mr McKeefery bringing this issue to us," a statement said. "It occurred not as a result of a NatWest error but we were able to help resolve it as a result of his pro-activity.
"The delay he experienced... was due, in part, to processes that need to be followed to return funds to the sending bank, though we also could have done more for him in moving the funds to a temporary suspense account."
NatWest left Mr McKeefery with £210 interest from the sum, which he says he has spent already.
"Having £210 isn't bad I guess for the hassle. I can't even remember what I spent it on."
A man was "shocked and amazed" when he found more than £1m in his bank account after an error.