You can find your Medicare ID number on your Medicare card. The number is printed under your name and to the left of your sex, according to Medicare.gov.
If you do not have your Medicare card, you can request a replacement card on the Social Security Administration's website, explains the U.S. Department of Health and Human Services. The site also provides a telephone number to request a new card over the phone rather than online. To request a new card, you need to provide your last exact payment amount or the month and year of your last payment along with your name, date of birth, social security number and telephone number.