Research by the House of Commons Library, obtained by the SNP, shows 50,740 Scottish pensioners have lost benefits worth a total of £90 million since 2010 under changes to Savings Credit.
Many more are set to lose out altogether when the benefit is scrapped for all new pensioners in 2016.
The changes affect Pension Credit, which was introduced in 2003 by Chancellor Gordon Brown to ensure all pensioners had a minimum income to live on.
The benefit is made up of two parts, the first of which, Guarantee Credit, tops up pensioners' weekly income if it falls below a certain level, currently £148.35 for a single person.
The second part, Savings Credit, is an extra payment for those who have saved towards retirement.
It is worth up to £20.52 a week to a single pensioner and £27.09 a week to a couple.
But Dr Eilidh Whiteford, the SNP's spokesman for Work and Pensions, said: "This benefit cut is hitting poorer pensioners who have planned ahead and worked hard for their retirement. It's a real shame they are being hit in this way."
The figures cover the period May 2010 to November 2013. On average the elderly lost £4.68 a week, or about £243 a year.
The Scottish Government's White Paper on independence says pensioners would get a guaranteed pension of £160 a week from 2016-17 thanks to a triple lock that ensures the state pension goes up by the highest of inflation, wages or 2.5 per cent - and the continuation of Savings Credit after a Yes vote.
Pensions Minister Steve Webb said: "The very reason the Triple Lock exists, and the state pension is at its highest level relative to earnings since 1992, is because this UK Government brought it in.
"So it is disingenuous and absurd for anyone to cite it as a reason to support independence."