The Liberal Democrat manifesto will include a commitment to keep the "triple lock" protection for state pensions.
But the party will strip wealthier pensioners of the winter fuel allowance, at a cost of up to £300 a year to older people.
An estimated 600,000 pensioners with annual incomes above about £45,000, 5% of those under 80 and 2% over over-80s, would lose the allowance under the Lib Dem plans, saving the Treasury around £105 million.
But the party said all of those receiving the full state pension would enjoy a rise from £122.30 to at least £137.15 a week by 2021, the equivalent of an extra £772 a year.
Labour has also pledged to retain the triple lock, which guarantees the state pension will rise in line with inflation, average earnings or 2.5%, whichever is the highest.
But speculation is rife Conservatives will ditch the guarantee, after Prime Minister Theresa May declined opportunities to confirm it would feature in her manifesto for the June 8 general election.
Lib Dem former business secretary Sir Vince Cable said the promise showed Lib Dems would help pensioners, when Conservatives would not.
But he is likely to come under fire over the winter fuel allowance from Labour, which earlier this year promised to keep the benefit, worth between £100 and £300 tax-free a year towards the heating bills of all over-65s, regardless of their wealth.
Setting out the plans, Sir Vince said: "Liberal Democrats believe that an important test of a civilised society is the way in which it cares for the elderly.
"We will protect the triple lock, unlike the Conservatives.
"The guiding principle of the pensions system must be to ensure that none are left unable to meet their basic needs for survival and participation in society, and that everyone is treated with the respect and dignity they deserve.
"A Lib Dem pensions minister introduced the triple lock guarantee to protect the state pension during coalition.
"We delivered on our manifesto commitment to increase the basic state pension by whatever is highest out of CPI inflation, average earnings or 2.5%."
The triple lock was a feature of the Liberal Democrat manifesto in 2010 and a key demand in negotiations on the formation of the coalition government with Conservatives.
Since its introduction in 2010, it has seen pensioner incomes rise faster than average earnings, sparking warnings from the independent Institute for Fiscal Studies that it will swallow up an ever greater share of state spending.
A cross-party House of Commons select committee report earlier this year described it as "inherently unsustainable" and recommended it should not be continued beyond 2020.
A Conservative spokesman said: "Because of the strong economy we have delivered, Theresa May and her Conservative team have increased the basic state pension by £1,250.
"The real risk to pensions comes from Jeremy Corbyn propped up in a coalition of chaos by the Lib Dems and the SNP."
A Labour Treasury spokesman said: "You can't trust the Liberal Democrats.
"They broke their promises and would do it again.
"Only in September their former leader Nick Clegg called for the triple lock to be dropped.
"For a party with so few MPs, they cannot agree with each other on anything, as we saw with tuition fees in the last parliament, you just cannot trust what the Lib Dems say."