〔Abstract〕 To investigate the mechanism through which miR‑21 improves chronic renal fibrosis in mice via targeted modulation of the phosphatase and tensin homolog (PTEN)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway. Methods Thirty‑two chronic kidney disease model mice were randomly divided into four groups (n=8 each group): model group, miR‑21 overexpression group, miR‑21 inhibition group, and miR‑21 inhibition + MK‑2206 group. Eight healthy mice were included as the control group. The miR‑21 overexpression, miR‑21 inhibition, and miR‑21 inhibition + MK‑2206 groups received tail‑vein injections of lentivirus (50 μL, 1×10⁸ TU per mouse) once weekly for three weeks. The control and model groups were injected with an equal volume of empty vector (LV‑NC). The miR‑21 inhibition + MK‑2206 group additionally received gavage of the AKT/mTOR pathway inhibitor MK‑2206 (480 mg/kg) once weekly for three weeks. The expressions of miR‑21, 24‑h urinary protein, serum creatinine (Scr), blood urea nitrogen (BUN), and renal tissue levels of collagen I, collagen III, α‑smooth muscle actin (α‑SMA), and PTEN protein, as well as p‑AKT/AKT and p‑mTOR/mTOR ratios, were compared among groups. HE staining was used to observe pathological changes in renal tissue, and Masson staining was used to observe the degree of renal fibrosis. A dual‑luciferase assay was performed to verify the targeting relationship between miR‑21 and PTEN. Results Compared with the model group, miR‑21 expression in renal tissue increased in the miR‑21 overexpression group (P<0.05) and decreased in the miR‑21 inhibition group (P<0.05). Compared with the model group, the miR‑21 overexpression group showed increased 24‑h urinary protein, Scr, BUN, and renal tissue expression of collagen I, collagen III, and α‑SMA (all P<0.05), while these indicators decreased in the miR‑21 inhibition group (all P<0.05). Compared with the miR‑21 inhibition group, the miR‑21 inhibition + MK‑2206 group exhibited lower 24‑h urinary protein, Scr, BUN, and renal tissue expression of collagen I, collagen III, and α‑SMA (all P<0.05). Compared with the model group, the miR‑21 overexpression group showed decreased PTEN protein expression (P<0.05) and increased p‑AKT/AKT and p‑mTOR/mTOR ratios (P<0.05), while the miR‑21 inhibition group showed increased PTEN expression (P<0.05) and decreased p‑AKT/AKT and p‑mTOR/mTOR ratios (P<0.05). Compared with the miR‑21 inhibition group, the miR‑21 inhibition + MK‑2206 group had lower p‑AKT/AKT and p‑mTOR/mTOR ratios (P<0.05), with no significant difference in PTEN protein expression (P>0.05). HE and Masson staining showed normal kidney structure and almost no fibrosis in the control group. The model group exhibited glomerular enlargement, capillary loop adhesion, and focal fibrosis. The miR-21 overexpression group showed severe destruction of glomerular structure, accompanied by extensive fibrosis and renal tubular atrophy. The pathological changes and degree of fibrosis were alleviated in the miR-21 inhibition group. The miR-21 inhibition + MK-2206 group showed only mild pathological changes and mild fibrosis, with the interstitium being largely normal. Compared with PTEN-WT + NC mimics 1, the relative luciferase activity in the PTEN-WT + miR-21 mimics group decreased (P<0.001). There was no statistically significant difference in relative luciferase activity between PTEN-WT + NC mimics group and PTEN-MUT + miR-21 mimics group (P>0.05). Conclusion miR‑21 may improve renal function indicators and alleviate renal fibrosis in chronic kidney disease mice via targeted modulation of PTEN and subsequently inhibiting theAKT/mTOR pathway.