Clothing retailers, beware. Amazon is expected to make a big leap in the apparel market this year.

According to Morgan Stanley, the e-commerce giant will become the top player of the U.S. apparel industry in 2018, having gained 1.5 percent of market share last year. The gains will largely be driven by millennials moving dollars away from bricks and mortar, the firm said in a new report.

Amazon has found the most success in selling casual items for everyday wear, while brands like Nike and Calvin Klein are helping bolster the platform with their popular merchandise.

"Amazon's online pure-play model is becoming increasingly critical for many brands to maintain overall market share," analyst Brian Nowak wrote in a note to clients. "Brands are plugging the department store 'leaky bucket' hole with growth on Amazon.com."

To date, Amazon trails only Walmart to claim the top spot among other apparel retailers Target, Kohl's and TJ Maxx.