NEW YORK- U.S. robo-adviser Betterment  said on Wednesday it is rolling out personalized portfolio management for clients with more than $100,000, marking the firm's first step away from the automatically designed and rebalanced portfolios that have become its hallmark.

Betterment Chief Executive Jon Stein said it is a move to appeal to wealthier clients who keep only a portion of their assets with Betterment, and who have said they would invest more if they could pick and choose what is in their portfolios.

"This is an addition for those clients who know that they need to adjust their portfolios to (compliment investments) they have externally," Stein said in a telephone interview.

Stein said the firm would continue to focus on retail investors who choose to invest in the firm's recommended portfolios. That client is typically around the age of 37 and has roughly $40,000 invested with Betterment.

"The vast majority of our clients still make their investments based on our recommendations and our smart defaults," said Stein, referring to the firm's signature robo-adviser service.

One of the earliest and largest U.S. robo-adviser startups, Betterment had roughly 270,000 customers and $10 billion in assets under management as of July 2017.

The firm has continued to add services that break out from its original model, including the move last year to add human financial advisors who work alongside the firm's investing algorithms.

(Reporting By Elizabeth Dilts; editing by Diane Craft) ((elizabeth.dilts@thomsonreuters.com; (646) 223-5063; Reuters Messaging: elizabeth.dilts.thomsonreuters.com@reuters.net))