You sign up for Medicare Part B either through Social Security (most common) or, if you worked for the railroad, through the Railroad Retirement Board, and you must do it during the right enrollment window to avoid penalties.

Main ways to sign up

  • Online (fastest option) :
    • Go to the Social Security website and apply for Medicare (Part A, Part B, or both).
    • If you already have Part A, choose the option to add Part B only.
* You’ll create or log into a Social Security account, answer eligibility questions, and submit the application electronically.
  • By phone :
    • Call Social Security at 1‑800‑772‑1213 (TTY: 1‑800‑325‑0778) Monday–Friday, typically 8 a.m.–7 p.m. local time.
* Tell them you want to enroll in **Medicare Part B** , and they can take your application or mail you the forms.
  • By mail or in person (using forms) :
    • Complete form CMS‑40B (Application for Enrollment in Medicare Part B).
* Mail or fax it to your local Social Security office, or drop it off in person if that office allows in‑person submissions.
  • If you or a spouse worked for a railroad :
    • Contact the Railroad Retirement Board at 1‑877‑772‑5772 or visit their website; they handle your Medicare enrollment instead of Social Security.

When you can sign up

  • Initial Enrollment Period (IEP) :
    • A 7‑month window: starts 3 months before the month you turn 65, includes your birth month, and ends 3 months after.
* Enrolling in this window usually avoids late penalties and coverage gaps.
  • Special Enrollment Period (SEP) :
    • If you delayed Part B because you had qualifying employer or union group health coverage, you can usually enroll later without a penalty.
* Often this involves completing CMS‑40B plus employer form CMS‑L564 to prove creditable coverage and submitting them to Social Security.
  • General Enrollment Period (GEP) :
    • If you missed IEP and don’t qualify for a SEP, you can enroll between January 1 and March 31 each year.
* Coverage typically begins later in the year, and you may owe a permanent late enrollment penalty on your Part B premium.

Key tips and “gotchas”

  • Enroll as soon as you’re eligible unless you’re sure your current job-based coverage lets you delay Part B without penalties.
  • Have your Medicare number (if you already have Part A), contact information, and any employer coverage details ready before starting the application.
  • If you’re using forms for a SEP, Social Security often expects CMS‑40B and CMS‑L564 together; incomplete paperwork can delay your Part B start date.

TL;DR: The easiest way to handle how do you sign up for Medicare Part B is to apply online through Social Security during your 7‑month initial window at 65, or during a special or general enrollment period, using form CMS‑40B (and CMS‑L564 if you’re coming off employer coverage).

Information gathered from public forums or data available on the internet and portrayed here.