The left-hander threw 25 pitches in the bullpen before Cincinnati opened a series against the Pittsburgh Pirates. The Reds will gradually increase his activities and hope to have him back sometime in May.
Chapman sustained fractures above his left eye and his nose when he was hit by Salvador Perez's line drive on March 19 during a relief appearance against Kansas City. He had a plate inserted in the forehead the next day. Chapman has to be careful not to get hit in the forehead while it heals, so he's restricted to working out and throwing on his own.
General manager Walt Jocketty and manager Bryan Price watched his bullpen session on Monday.
"He threw all of his pitches, including his slider," Price said. "He was sharp, extremely enthusiastic and happy to be out there, as we all were. It was exciting."
There are a lot more steps before he can get back into a game. He'll throw in the bullpen a few more times before the Reds allow him to pitch to some of their hitters.
"It was an obvious and essential step to the next thing, which will be live batting practice and game situations," Price said. "I think facing batters will be the biggest hurdle for him initially, but we haven't seen any reason to feel like he's other than optimistic about getting back on the mound."
Chapman had to rest completely for weeks after the surgery. He said it felt good to get back on a mound for the first time in nearly four weeks.
"Everything went well," Chapman said, with assistant trainer Tomas Vera translating. "My fastball was really good. My command was good. The breaking ball worked. Everything was great."
Price said that Chapman will have the option of using a batting practice screen in front of the mound the first time he throws to hitters.
"It will really end up with where he's comfortable," Price said. "We'll see. The big test will be when he gets back into the game, facing live competition."
Chapman is one of eight Reds who opened the season on the disabled list, the most in the majors. Reliever Jonathan Broxton and catcher Devin Mesoraco were activated last week.
Left-handed reliever Sean Marshall is scheduled to make back-to-back appearances for Triple-A Louisville this week, a good test of whether his shoulder has fully recovered from the soreness he felt during spring training. If there's no problem, he could be activated later in the week.
"We want to get him through these outings," Price said. "Going back-to-back for the first time is a step in the right direction - a huge step in the right direction, actually. We'll see how he comes out of it."