Walker was joined by right-hander Virgil Vazquez, who will pitch out of the bullpen after starting nine games for the Pirates earlier this season.
Pirates manager John Russell said Walker will likely be used off the bench and in pinch-hitting roles.
"I don't know how much playing time he'll get, but we'll continue his development here," Russell said. "
Walker hit .264 with 31 doubles, 14 homers and 69 RBIs in 95 games for the Indians. In August, he hit .339 with four homers, 29 RBIs and 14 doubles.
"I'm really happy with the way things turned out in August," Walker said. "That's what I'm capable of. The way I felt, the way my swing felt and my mentality is what propelled me through August."
Walker said a turning point for him was when he sprained a PCL ligament in his knee June 8, forcing him to miss six weeks.
"It put things in perspective from a baseball standpoint," Walker said. "When I got healthy and got back, I wasn't going to let any outside influence affect me the last month and a half. I wanted to make a concerted effort to bust my butt as hard as I could."
Before the injury, it looked as if Walker would not receive a September callup given his struggles to find consistency at the plate. He had also expressed doubts about his future in the organization after the Pirates acquired a slew of new prospects. But a hot month of August halted that, and Walker said he's committed to the club.
"Some things may have been said in the heat of the moment," Walker said. "I kind of felt like I was [spinning] my wheels in the organization."
"I'd like to be with this organization for a long time. That's really how I feel, and I'm hoping to take advantage."
Walker, now 23, was selected 11th overall in the 2004 First-Year Player Draft out of Pine-Richland High School in Pittsburgh. He was originally drafted as a catcher but converted to third base in 2007. He is the son of former Major League pitcher Tom Walker and expected his parents to be in attendance for Tuesday's game against the Reds.
"I think this is where obviously I want to be," Walker said. "I get a chance to get back home and play in front of the hometown fans. It's something I've dreamed about since I've been seven or eight years old."
Yet Walker isn't satisfied with just being on the Pirates. He wants to help move the team in the right direction.
"For me it's not about that I made it here," Walker said. "It's so much more than that. I want to help this team win as soon as possible. That's really where I am right now."