CHICAGO — Once a dormant area of empty warehouses, the River North section of Chicago has evolved into a nexus of dining, night life and, most recently, an aspiring rival to Silicon Valley. Its 45 square blocks are home to the headquarters of Groupon, the Chicago offices of Google and several hundred technology start-ups.

Now River North’s digital transformation is extending to one of the neighborhood’s most storied — and decidedly low-tech — commercial addresses. The Merchandise Mart, a Depression-era behemoth of limestone, concrete and steel that has long been synonymous with fabric bolts and furniture, is becoming a destination for the city’s digital set.

“River North as an area has become very tech-savvy and very tech-cool,” said Todd O’Hara, founder and chief executive of Toodalu, an app-building start-up that moved into the building this year. “The Merchandise Mart is definitely kind of the pinnacle of all of it because of everyone coming in.”

The biggest newcomer, Motorola Mobility, plans to relocate its headquarters from the suburb of Libertyville to four floors of the mart next year, as well as take up a big chunk of the building’s roof space for entertaining and group events.