The Manx Pension Supplement is to be gradually phased out in order to save money, the government has said.
Under the current system, about 80% of pensioners have their weekly payments boosted by £55 to £175.
The government said this supplement, which annually costs £36m, will be reduced by 5% a year over 20 years.
The state pension age is also set to rise from 65 to 67 by 2019, while five extra years' National Insurance contributions will need to be made.
The reforms, approved by Tynwald and due to be introduced in April 2019, are needed to make the public finances "more sustainable", the government said.
A spokesman said the changes will protect state pension payments for "at least the next 70 years".
Without the reforms, he said the Manx National Insurance fund will run dry by 2053.
The state pension age will increase to 66 for those born between 6 April 1960 and 5 March 1961.
People born after 5 March 1961 but before 6 April 1977 will have to wait until they are 67 until they are eligible for a state pension.
It is currently paid to about 19,000 people, a number expected to rocket by 75% by 2035 as the population ages.