China would account for US$2 trillion of that amount, the company said, after the Asian giant had become the first market to reach US$1 trillion in online retail sales in 2018.
Forrester said India was the world's fastest growing region, with a CAGR of 25.8%, and it would reach US$85 billion in online retail sales by 2023.
"Mobile accounts for 72% of online retail sales in Asia Pacific," the company said. "Smartphone usage is the key driver, especially in China and India.
The Forrester study found that online fashion, including apparel and footwear, was the biggest category in Asia Pacific, accounting for US$232 billion in online retail sales in 2018.
Groceries were the fastest growing category, with China remaining the largest market for online grocery, followed by Japan and South Korea.
"We expect online grocery [sales] to grow at a CAGR of 26.5% to reach US$260 billion by 2023," Forrester added.