The Pirates have not confirmed the move. General manager Neal Huntington said late Wednesday afternoon that the official announcement will come following Thursday's game.
Barthmaier had been scheduled to start for Triple-A Indianapolis on Thursday, making it necessary to inform the right-hander ahead of time. This will be his first Major League callup.
Barthmaier started the season at Double-A Altoona, where he went 2-4 with a 4.86 ERA in 10 starts. The right-hander earned a promotion to Triple-A in late May when Bryan Bullington was called up to the Pirates.
In five starts with Indianapolis, Barthmaier went 2-1 and allowed 10 earned runs in 31 2/3 innings. His most recent start came on Saturday, when he limited Rochester to one earned run on four hits, while striking out eight in six innings of work.
The Pirates were in need of fill-in starters for both Friday and Saturday because of injuries to starters Phil Dumatrait and Ian Snell.
Though Barthmaier, whom the Pirates claimed off waivers last fall, is already on the club's 40-man roster, the Pirates will have to make a corresponding roster move in order to make space for him on the team's 25-man roster. That move is expected to be announced on Thursday night.
Jenifer Langosch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.