Tetragonia tetragonoides (Pall.) Kuntze (TTK) is a groundcover found along coastal areas of the Korean peninsula. TTK is traditionally used to improve women's health and treat gastrointestinal diseases. Use of herbal medicines in the treatment of mood disorders has recently been suggested as an alternative therapeutic strategy. In the present study, we determined that consumption of TTK extract ameliorated progression of depressive-like symptoms in ovariectomized (OVX) rats and further examined the mechanisms involved, i.e., synthesis, release, and reuptake(s) of serotonin (also known as 5-HT). We assessed the mRNA expression levels of tryptophan hydroxylases (TPH-1 and TPH-2) and serotonin transporter (SERT) as well as the reuptake activity of serotonin in RBL-2H3 cells. We also determined whether or not TTK extract regulates the serum level of serotonin and improves depressive-like symptoms in 0.5, 1, and 2% TTK-fed OVX female rats in a forced swimming test. Our results show that the mRNA levels of TPH-1 and SERT were significantly reduced, whereas the mRNA level of TPH-2 was dose-dependently elevated by TTK (50 and 100 μg/mL) in RBL-2H3 cells. TTK significantly inhibited LPS- (lipopolysaccharide-) induced serotonin uptake in RBL-2H3 cells in a dose-dependent manner. The serum level(s) of serotonin was elevated by 1% and 2% TTK treatment in OVX female rats. Moreover, immobility time in the forced swimming test was reduced by 1% and 2% TTK treatment but not altered by 0.5% TTK treatment in OVX female rats. Taken together, these results indicate that TTK may significantly inhibit depressive-like symptoms due to upregulation of serotonin level(s) and regulation of serotonin reuptake activity. Thus, TTK may exert beneficial effects on depression during pre- or/and postmenopausal periods via modulation of serotonin synthesis and metabolism.