Yes.  If a Medicare beneficiary is outside of the annual Open Enrollment Period (AEP) or the Medicare Advantage OEP, they can use a Special Enrollment Period (SEP) to join a Medicare Part D plan (PDP) or Medicare Advantage plan (MA or MAPD) if they lose or leave their Employer or Union Group Health Plan (EGHP).

A Medicare beneficiary also will be granted a Special Enrollment Period to leave (disenroll from) their Medicare Advantage plan or Medicare Part D plan and join - or re-join their EGHP.

The Employer/Union Group Health Plan Medicare Advantage or Medicare Part D Special Election Period (SEP)

As reference, the Medicare Advantage and Part D Enrollment and Disenrollment Guidance,  Section 30.6.16 – SEP for individuals who elect into or out of employer sponsored coverage, notes:

This [Special Election Period] SEP allows eligible individuals to do the following:

• Enroll in an employer or union sponsored MA or Part D plan;
  
  
• Disenroll from an employer sponsored MA plan or Part D plan (i.e., Employer Group Health Plan (EGHP)) and enroll in a different MA or Part D plan;
  
  
• Disenroll from any employer-sponsored coverage (including Consolidated Omnibus Budget Reconciliation Act (COBRA)) to enroll in an MA or Part D plan.
  
  
• Disenroll from an MA plan or Part D plan to take employer sponsored coverage of any kind.
  
  

This SEP is available to individuals who have enrolled in or are enrolling in an employer or union sponsored MA or Part D plan. The SEP ends two months after the month the employer or union coverage of any type ends or when used to disenroll from an MA or Part D plan. This SEP typically begins when the EGHP allows the individual to make changes to their coverage, such as during the employer's or union's "open season."

NOTE: An individual using this SEP may request an effective date of up to three months after the month in which the election is made; however, the effective date may not be earlier than the first day of the month following the month in which the election was made or the first day of the individual's entitlement to Medicare Parts A and B. [emphasis and formatting added]

Important:  How long does the Special Enrollment Period last?

As noted above, the SEP continues up to two months after the end of employer health coverage to join a Medicare Part D or Medicare Advantage plan.  Medicare beneficiaries who did not enroll in Medicare Part D or Medicare Part B (or Premium-Medicare Part A) when first eligible because they were covered under a group health plan are granted a Special Enrollment Period for Medicare Part A, Medicare Part B, and Medicare Part D prescription drug coverage if they no longer have employer or group health coverage.

Example: Leaving your employer health plan and joining a Medicare plan.

Medicare beneficiaries can enroll in a Medicare Part D or Medicare Advantage plan anytime while covered under the group health plan based on current employment, or up to two months after the employment ends or employer coverage ends or the group health plan coverage ends, whichever comes first.

Related:  The Medicare Part B Special Election Period (SEP)

Medicare beneficiaries who qualify for a Special Enrollment Period (SEP) can enroll in Medicare Part B without incurring a late-enrollment penalty while they are still employed (or their spouse is still employed and up to eight (8) months after they no longer have employer coverage, change employer coverage, or are no longer working.

As reference, the Medicare.gov site states:

Special Enrollment Period (SEP) for the Working Aged and Working Disabled.

Individuals who do not enroll in Part B or premium Part A when first eligible because they were covered under a group health plan based on their own or a spouse's current employment (or the current employment of a family member, if disabled) may enroll during the SEP. The individual can enroll at any time while covered under the group health plan based on current employment, or during the 8-month period that begins the month the employment ends or the group health plan coverage ends, whichever comes first.

NOTE previous to 2021: Individuals with ESRD [End-stage Renal Disease or kidney failure] are not eligible to enroll during the SEP.
Starting in 2021, Medicare beneficiaries with ESRD will be allowed to join a Medicare Advantage plan.

Sources include:

Medicare Advantage and Part D Enrollment and Disenrollment Guidance Updated: 2024, Section 30.6.16 – SEP for individuals who elect into or out of employer sponsored coverage

2024 and prior years:
Medicare Prescription Drug Benefit Manual, Chapter 3 - Eligibility, Enrollment and Disenrollment, Updated: August 19, 2011 (Revised:  August 15, 2023),
(https://www.cms.gov/Medicare/Eligibility-and-Enrollment/MedicarePresDrugEligEnrol)

Medicare Managed Care Manual, Chapter 2 - Medicare Advantage Enrollment and Disenrollment, Updated: August 19, 2011 (Revised: August 15, 2023),
(https://www.cms.gov/Medicare/Eligibility-and-Enrollment/MedicareMangCareEligEnrol)

https://www.cms.gov/ medicare/ eligibility-and-enrollment/ origmedicarepartabeligenrol/ index.html or https://www.medicare.gov/ sign-up-change-plans/ get-parts-a-and-b/ when-sign-up-parts-a-and-b/ when-sign-up-parts-a-and-b.html and https://www.govinfo.gov/content/pkg/FR-2020-06-02/pdf/2020-11342.pdf )

View a list of other situations causing a Special Election Period (SEP)