A Study to Learn More About the Effects and Safety of Felzartamab Infusions in Adults With Primary Membranous Nephropathy (PMN)

Key Inclusion Criteria: * Diagnosed with PMN in need of IST according to the Investigator's clinical judgment. The diagnosis of PMN must be documented with the presence of nephrotic syndrome, and hypoalbuminemia, and confirmed with a kidney biopsy either during Screening or within 5 years of signing the informed consent form (ICF) \[see kidney biopsy exception below for participants positive for anti-PLA2R antibodies\]. For these participants, the biopsy report with redacted protected health information must be available to be reviewed by the Sponsor or an independent nephropathologist. If the participant requires a kidney biopsy during Screening, medical monitor approval must be obtained, and all other eligibility criteria should be reviewed to ensure that the participant is otherwise eligible prior to performing the kidney biopsy. a. Kidney biopsy exception for anti-PLA2R antibody positive participants: Participants who are positive for anti-PLA2R antibodies at Screening and have not had a kidney biopsy performed within 5 years of signing the ICF may be eligible for the study without undergoing a kidney biopsy based on medical monitor review of screening tests confirming normal estimated glomerular filtration rate (eGFR), presence of nephrotic syndrome, hypoalbuminemia, positive anti-PLA2R antibody test (defined as an anti-PLA2R antibody titer ≥ 20 RU/mL), and documentation provided by the Investigator that the work-up for secondary causes of membranous nephropathy (MN) was negative with no identifiable secondary causes. * Meets one of the following: 1. Newly diagnosed PMN, defined as having never received IST for PMN in the past. 2. Relapsed PMN, defined as documented achievement of CR or partial remission (PR) after treatment with an IST for PMN followed by reappearance of nephrotic range proteinuria (urine protein to creatinine ratio \[UPCR\] ≥ 3.0 gram per gram \[g/g\] from a 24-hour urine collection or proteinuria ≥ 3.5 gram per 24 hour \[g/24 h\]). * Participants must be on the maximally approved dose or maximally tolerated dose of at least one renin-angiotensin-aldosterone system (RAAS) inhibition therapy (e.g. angiotensin-converting enzyme inhibitor \[ACEI\], angiotensin receptor blocker \[ARB\], and/or mineralocorticoid receptor antagonists \[MRA\]) for at least 4 weeks prior to Screening. Participants not on the maximally approved dose of RAAS inhibition may be enrolled provided there is documented intolerance to maximal RAAS inhibition (e.g., angioedema, development of postural hypotension, lightheadedness, hyperkalemia, etc). Participants on MRA therapy alone at Screening must have documented prior intolerance of ACEI or ARB therapy. Participants who do not tolerate ACEI or ARB therapy are not required to start MRA therapy prior to Screening. * A UPCR of ≥ 3.0 g/g (as determined by a 24-hour urine collection) or total proteinuria ≥ 3.5 g/24 h (as determined by a 24-hour urine collection) at Screening after best supportive care for at least 4 weeks prior to signing the ICF. Key Exclusion Criteria: * Secondary cause of MN (e.g., malignancies, medications, systemic lupus erythematosus \[SLE\], hepatitis B, hepatitis C, etc). * Severe renal impairment is defined as an eGFR ≤ 30 mL/min/1.73m\^2 at Screening or including the need for dialysis or renal replacement therapy. Note: Other protocol-defined Inclusion/Exclusion criteria may apply.