Polyethylene terephthalate nanoplastics (PET-NPs) are utilized in the production of medical bionic materials and the packaging of beverages. Betaine is a ubiquitous natural constituent present in organisms such as plants, animals, and microorganisms. So, the current investigation tries to find out if PET-NPs could seriously harm mice's kidneys and whether betaine could have any ameliorative effects. In this study, a total of 40 mice were separated into four groups (ten mice in each): Group I (performed as the control group), Group II (received 1000 mg/kg betaine intraperitoneally), Group III (received 200 mg/kg PET-NPs orally), and Group IV (was given betaine first, and after 1 hour, PET-NPs were given at dosages that were the same as those given to groups II and III, respectively) daily for a month. Serum and kidney samples were collected and processed for biochemical and histological assessments. The current study found that PET-NPs significantly increased blood urea nitrogen (BUN), creatinine, and malondialdehyde levels (MDA), while reducing glutathione (GSH) levels. The histological examination revealed multiple histopathological alterations. The PET-NPs-exposed group demonstrated renal corpuscle hypotrophy, a loss of cellular structure in some proximal convoluted tubules (PCT) and distal convoluted tubules (DCT). The renal medulla exhibits hyalinization, congestion, and degeneration of collecting tubules. Conversely, the pre-administration of betaine results in a decline in BUN, creatinine, and MDA concentrations. Furthermore, there is a rise in GSH levels, and the group pretreated with betaine showed significant improvement in kidney architecture, with the renal cortex showing almost normal architecture and the collecting tubules in the renal medulla slightly improving. In conclusion, betaine showed a promising nephroprotective effect against PET-NP-induced toxicity