ImportanceBefore 2021, most Medicare beneficiaries with end-stage renal disease (ESRD) were unable to enroll in private Medicare Advantage (MA) plans. The 21st Century Cures Act permitted these beneficiaries to enroll in MA plans effective January 2021.ObjectiveTo examine changes in MA enrollment among Medicare beneficiaries with ESRD after enactment of the 21st Century Cures Act overall and by race or ethnicity and dual-eligible status.Design, Setting, and ParticipantsThis cross-sectional time-trend study used data from Medicare beneficiaries with ESRD (both kidney transplant recipients and those undergoing dialysis) between January 2019 and December 2021. Data were analyzed between June and October 2022.Exposures21st Century Cures Act.Main Outcomes and MeasuresPrimary outcomes were the proportion of Medicare beneficiaries with prevalent ESRD who switched from traditional Medicare to MA between 2020 and 2021 and those with incident ESRD who newly enrolled in MA in 2021. Individuals who stayed in traditional Medicare were enrolled in 2020 and 2021 and those who switched to MA were enrolled in traditional Medicare in 2020 and MA in 2021.ResultsAmong 575 797 beneficiaries with ESRD in 2020 or 2021 (mean [SD] age, 64.7 [14.2] years, 42.2% female, 34.0% Black, and 7.7% Hispanic or Latino), the proportion of beneficiaries enrolled in MA increased from 24.8% (December 2020) to 37.4% (December 2021), a relative change of 50.8%. The largest relative increases in MA enrollment were among Black (72.8% relative increase), Hispanic (44.8%), and dual-eligible beneficiaries with ESRD (73.6%). Among 359 617 beneficiaries with TM and prevalent ESRD in 2020, 17.6% switched to MA in 2021. Compared with individuals who stayed in traditional Medicare, those who switched to MA had modestly more chronic conditions (6.3 vs 6.1; difference, 0.12 conditions [95% CI, 0.10-0.16]) and similar nondrug spending in 2020 (difference, $509 [95% CI, −$58 to $1075]) but were more likely to be Black (difference, 19.5 percentage points [95% CI, 19.1-19.9]) and have dual Medicare-Medicaid eligibility (difference, 20.8 percentage points [95% CI, 20.4-21.2]). Among beneficiaries who were newly eligible for Medicare ESRD benefits in 2021, 35.2% enrolled in MA.Conclusions and RelevanceResults suggest that increases in MA enrollment among Medicare beneficiaries with ESRD were substantial the first year after the 21st Century Cures Act, particularly among Black, Hispanic, and dual-eligible individuals. Policy makers and MA plans may need to assess network adequacy, disenrollment, and equity of care for beneficiaries who enrolled in MA.