When Can I Sign Up For A Medicare Supplement? 
Assuming you are not under 65 and disabled , you can apply any time you are at least 65 years old and enrolled in both Medicare Part A and Part B -LRB- original Medicare -RRB- . If you are on a Medicare Advantage Plan , you may not purchase a Medicare Supplement unless you switch back to original Medicare . The best time to `` sign up '' for a Medicare Supplement or `` Medigap '' policy is during your Medigap Open Enrollment Period , which lasts for 6 months and begins on the first day of the month your both 65 or older and enrolled in Medicare Part B . During your Medigap Open Enrollment Period , an insurance company can not use medical underwriting and can not rufue to sell you any Medigap policy it offers . 