Long noncoding RNA (lncRNA) have critical roles in various pathophysiological processes, and are frequently dysregulated in many diseases, particularly in cancer. The lncRNA glypican 3 antisense transcript 1 (GPC3‐AS1) has been reported to be a potential biomarker for hepatocellular carcinoma (HCC) screening. However, the exact biological functions of GPC3‐AS1 in HCC, and its roles and regulation mechanisms regarding GPC3 are still unknown. In this study, we observed a significant upregulation of GPC3‐AS1 in HCC. Increased expression of GPC3‐AS1 was associated with α‐fetoprotein, tumor size, microvascular invasion, encapsulation, Barcelona Clinic Liver Cancer stage, and worse prognosis of HCC patients. Furthermore, we found that GPC3‐AS1 physically associated with P300/CBP‐associated factor and recruited it to the GPC3 gene body region, consequently inducing an increase in euchromatic histone marks and activating GPC3 transcription. GPC3‐AS1 expression was strongly correlated with GPC3 in HCC tissues. Gain‐of‐function and loss‐of‐function analyses showed that GPC3‐AS1 overexpression enhanced HCC cell proliferation and migration in vitro and xenograft tumor growth in vivo. GPC3‐AS1 knockdown inhibited HCC cell proliferation and migration. Moreover, the effects of GPC3‐AS1 on HCC cell proliferation and migration were dependent on the upregulation of GPC3. Collectively, our studies indicate that GPC3‐AS1 significantly promotes HCC progression via epigenetically activating GPC3, and identifies GPC3‐AS1 as a potential therapeutic target for HCC. The long noncoding RNA GPC3‐AS1 is significantly upregulated in hepatocellular carcinoma (HCC) and indicates poor prognosis of HCC patients. GPC3‐AS1 physically associates with P300/CBP‐associated factor (PCAF) and recruits it to the GPC3 gene body region, consequently inducing an increase in euchromatic histone marks and activating GPC3 transcription. GPC3‐AS1 overexpression enhances HCC cell proliferation and migration in vitro and xenograft tumour growth in vivo.