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 Part D and Medicare Advantage plan manuals note:

A [Special Election Period] exists:

-- for individuals making an [PDP or MA] enrollment request into or out of an employer/union group-sponsored Part D [or MA] plans,
-- for individuals disenrolling from a Part D [or MA] plan to take employer/union-sponsored coverage of any kind, and
-- for individuals disenrolling from employer/union-sponsored coverage (including COBRA coverage) to enroll in a Part D [or Medicare Advantage] plan.

The SEP EGHP may be used when the EGHP [Employer/Union Group Health Plan] allows the individual to make changes to their plan choices, such as during the employer’s or union’s “open season,” or at other times the employer or union allows.

This SEP is available to individuals who have (or are enrolling in) an employer or union plan and ends 2 months after the month the employer or union coverage ends.

The individual may choose the effective date of enrollment or disenrollment, up to 3 months after the month in which the individual completes an enrollment or disenrollment request. However, the effective date may not be earlier than the first of the month following the month in which the request was made. The effective date also may not be earlier than the first day of the individual’s entitlement to Medicare. [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 Prescription Drug Benefit Manual, Chapter 3 - Eligibility, Enrollment and Disenrollment, Updated: August 19, 2011 (Revised:  November 16, 2011, August 7, 2012,August 30, 2013, August 30, 2014,July 6, 2015, September 1, 2015, September 14, 2015, December 30, 2015, May 27, 2016,August 25, 2016, June 15, 2017,July 31, 2018 & August 12, 2020),
(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: November 16, 2011, August 7, 2012, August 30, 2013,August 14, 2014, July 6, 2015, September 1, 2015, September 14, 2015, December 30, 2015, May 27, 2016, August 25, 2016, June 15, 2017, July 31, 2018 & August 12, 2020),
(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)