Recent studies have demonstrated that microRNAs (miRs) serve a crucial role during the development of gestational diabetes mellitus (GDM). However, the mechanisms underlying miR-345-3p and its protective role during GDM have not been previously reported. The present study investigated miR-345-3p expression and function in vitro, and the possible molecular mechanisms underlying GDM. Compared with healthy pregnant women, miR-345-3p was downregulated in the placental tissue and peripheral blood of patients with GDM. Further investigation revealed that BCL2-antagonist/killer 1 (BAK1) was a predicted target gene of miR-345-3p, and the expression of BAK1 was significantly increased in patients with GDM compared with healthy pregnant women. In vitro analysis revealed that miR-345-3p mimic significantly increased cell viability, migration and invasion, inhibited apoptosis, upregulated Bcl-2 and matrix metallopeptidase 9 expression, and decreased Bax expression compared with the control group. Furthermore, miR-245-3p mimic-induced alterations were reversed by BAK1 overexpression. The results suggested that miR-345-3p overexpression exhibited a protective role in patients with GDM by inhibiting HTR8-/SVneo cell apoptosis, and promoting cell proliferation and migration via targeting BAK1. The use of miR-345-3p for the diagnosis of GDM requires further investigation.