Prime Minister David Cameron has announced the changes following a long-running campaign for equal pension rights for partners of those serving in the Armed Forces.
Currently, war widows receiving pensions from schemes in place between 1973 and 2005 lose their entitlements if they co-habit or remarry.
But the new pension arrangements will ensure a spouse or civil partner of all members of the Armed Forces will retain their pension for life if they remarry after April 1 next year.
The scheme is expected to affect some 3,000 widows and cost an estimated £120m over 40 years.
Mr Cameron said it is “absolutely wrong” that under the current system some Armed Forces widows lose their pension if they choose to remarry.
“This means that people, who have made huge sacrifices for our country, have had to make an agonising choice between their financial security or loneliness,” he said.
“That’s why I was determined to put this right and to respond to the concerns of many who have campaigned for a long time on this issue.
“And I am delighted to announce that we will change the rules to ensure that when our Armed Forces widows find happiness with someone else they can keep their pension for life.
“This reflects our clear commitment to uphold the Armed Forces covenant which we enshrined in law.”