Google plans to announce this morning that Boston will be among the first six cities worldwide to offer predicted arrival times for transit on Google Maps, with MBTA bus data expected to go live on the desktop and mobile mapping service by midday.

Although other real-time prediction applications already exist for the T, the ubiquity of Google Maps will make the bus system more convenient for tens of thousands of riders, said MBTA general manager Richard A. Davey.

The announcement is a clear sign of the T’s dramatic shift in its approach to the tech community since 2008, when the MBTA guarded data so carefully that it sued a group of MIT undergraduates who uncovered flaws in the CharlieCard automated fare system, to keep them from presenting their findings at a conference.

And the T was nowhere to be found when Google first joined with transit agencies more than five years ago to incorporate schedules and station locations into Google Maps.

The MBTA was not added until 2009, after other major cities like New York and Chicago, a lag that prompted 1,500 people to join a “Put the MBTA on Google Transit!!!’’ Facebook group.

“For the T, it’s a little bit of a coming of age,’’ Davey said. “Now, we’re one of the first and certainly the largest [transit agency] in the United States’’ to work with Google on predictions.

In addition to Boston, the real-time predictions will begin today on Google Maps for San Diego, San Francisco, and Portland, Ore., as well as for Madrid and Turin, said Noam Ben-Haim, Google Maps product manager, in a phone interview from Zurich, where his development team is based.

He said they have been working to add predictive data to Google Maps for the past year, amid requests from riders and agencies alike.

About 200 million different mobile users now visit Google Maps at least once a month via Android, iPhone, BlackBerry, and other devices. That represents about 40 percent of all Google Maps searches, and mobile users are gaining on desktop surfers, said Anne Espiritu, Google spokeswoman.

Boston commuter rail and subway predictions will be next for Google Maps, though neither the company nor the Massachusetts Bay Transportation Authority revealed a target date.

But arrival predictions provide arguably the greatest benefit for bus customers, who outnumber commuter rail travelers more than 2 to 1, with nearly 400,000 daily riders spread out among nearly 200 routes and 8,000 stops.