Panax quinquefolium hairy root cultures synthesize triterpenoid saponins named ginsenosides, that have multidirectional pharmacological activity. The first rate-limiting enzyme in the process of their biosynthesis is 3-hydroxy-3-methylglutaryl CoA reductase (HMGR). In this study, a 741 bp fragment of the P. quinquefolium HMGR gene (PqHMGR), consisting of a proximal promoter, 5′UTR (5′ untranslated region) and 5′CDS (coding DNA sequence) was isolated. In silico analysis of an isolated fragment indicated a lack of tandem repeats, miRNA binding sites, and CpG/CpNpG elements. However, the proximal promoter contained potential cis-elements involved in the response to light, salicylic, and abscisic acid (ABA) that was represented by the motif ABRE (TACGTG). The functional significance of ABA on P. quinquefolium HMGR gene expression was evaluated, carrying out quantitative RT-PCR experiments at different ABA concentrations (0.1, 0.25, 0.5, and 1 mg·L−1). Additionally, the effect of abscisic acid and its time exposure on biomass and ginsenoside level in Panax quinquefolium hairy root was examined. The saponin content was determined using HPLC. The 28 day elicitation period with 1 mg·L−1 ABA was the most efficient for Rg2 and Re (17.38 and 1.83 times increase, respectively) accumulation; however, the protopanaxadiol derivative content decreased in these conditions.