Objectives. To explore the role of microRNA-21-5p (miR-21-5p) in hypoxia/reoxygenation- (H/R-) induced HT22 cell damage. Methods. The hypoxia/reoxygenation (H/R) model was established in mouse neuronal cells HT22. Cell Counting Kit-8 (CCK-8) and qRT-PCR were used to determine the effects of H/R treatment on cell viability and miR-21-5p expression. HT22 cells were transfected with miR-21-5p mimic or negative control (NC) followed by the induction of H/R; cell viability, apoptosis, and SOD, MDA, and LDH activities were detected. Besides, the apoptosis-related proteins including BAX, BCL2, cleaved caspase-3, and caspase-3 as well as proteins of EGFR/PI3K/AKT signaling pathways were measured by Western blot. To verify the target relation between cytoplasmic polyadenylation element binding protein 3 (CPEB3) and miR-21-5p, luciferase reporter gene experiment was performed. After cotransfection with miR-21-5p mimic and CPEB3 plasmids, the reversal effects of CPEB3 on miR-21-5p in H/R damage were studied. Results. H/R treatment could significantly reduce the cell viability (
P
<
0.05
) and miR-21-5p levels (
P
<
0.05
) in HT22 cells. After overexpressing miR-21-5p, cell viability was increased (
P
<
0.05
) under H/R treatment, and the apoptosis rate and the levels of apoptosis-related proteins were suppressed (all
P
<
0.05
). Furthermore, SOD activity was increased (
P
<
0.05
), while MDA and LDH activity was decreased (both
P
<
0.05
). Besides, miR-21-5p could restore the activation of the EGFR/PI3K/AKT signaling pathway inhibited by H/R treatment (all
P
<
0.05
). The luciferase reporter gene experiment verified that CPEB3 is the target of miR-21-5p (
P
<
0.05
). When coexpressing miR-21-5p mimic and CPEB3 in the cells, the protective effects of miR-21-5p under H/R were reversed (all
P
<
0.05
), and the activation of the EGFR/PI3K/AKT pathway was also inhibited (all
P
<
0.05
). Conclusion. This study showed that miR-21-5p may regulate the EGFR/PI3K/AKT signaling pathway by targeting CPEB3 to reduce H/R-induced cell damage and apoptosis.