The global administration of mRNA vaccines in response to the coronavirus disease 2019 (COVID-19) pandemic has been crucial in mitigating the spread of the virus. While these vaccines are generally safe and effective, there have been occasional reports of rare adverse effects, including new-onset nephropathies. Primary Sjögren's syndrome (pSS), an autoimmune disorder primarily affecting the exocrine glands, can also present with renal involvement, most commonly as tubulointerstitial nephritis (TIN). A 52-year-old female with a history of pSS developed shortness of breath, generalised edema, and oliguria 1 month after receiving her fourth dose of the COVID-19 mRNA vaccine. Initial evaluation revealed bilateral pleural effusion on chest X-ray. Laboratory evaluations revealed rapidly progressive glomerulonephritis (RPGN) and nephrotic syndrome. Renal biopsy findings showed mesangial expansion, focal crescent formation, pronounced tubulointerstitial nephritis, and positive staining for anti-phospholipase A2 receptor (PLA2R). The temporal association, coupled with renal biopsy findings, strongly suggested a vaccine-related trigger, and the diagnosis of new-onset primary membranous nephropathy (MN) following COVID-19 mRNA vaccination was made. The patient was treated with haemodialysis, plasma exchange, corticosteroid pulse therapy, and immunosuppressive agents, resulting in complete remission of proteinuria within 3 months. This case underscores the potential for COVID-19 mRNA vaccines to precipitate primary MN in patients with pre-existing familial autoimmune conditions such as pSS. It also emphasises the importance of recognising vaccine-related renal complications in autoimmune patients and the necessity for close monitoring and prompt intervention to prevent serious complications.