For decades, traditional plant medicines have been utilized to treat diabetes. In traditional medicine, Origanum majorana L., sometimes known as sweet marjoram, is used to cure a variety of illnesses. It belongs to the Lamiaceae family. The purpose of the current study was to look into the ameliorative and antihyperglycemic effects of O. majorana leaves extract (OMLE) on the splenic tissue of diabetic rats. The following four groups (n=10) of forty adult male albino rats were created; group (Gp1) was used as the negative control. For four weeks, Gp2 had been giving OMLE (20 mg/kg b.wt) orally every day. STZ (40 mg/kg b.wt) was injected intraperitoneally (i.p.) into Gp3 and Gp4. OMLE (20 mg/kg b.wt) was subsequently given orally to Gp4 every day for four weeks. Serum glucose, insulin levels, superoxide dismutase (SOD), glutathione reductase (GSH), catalase (CAT) activity, white blood cell count (WBCs), and malondialdehyde (MDA) level were measured. Evaluations of spleen tissue histological studies were made. Rats with diabetes mellitus had a noteworthy rise in blood glucose and MDA concentrations, accompanied by a noteworthy fall in plasma insulin levels, white blood cell count, SOD, GSH, and CAT activities (P < 0.01). The findings showed that the spleen tissue of diabetic rats had many histological alterations. The aforementioned biochemical indicators and histological studies significantly improved when diabetic rats were treated with OMLE. In conclusion, OMLE may have a protective and antidiabetic effect on the spleen tissues of diabetic rats.