Police say Reginald Riddle is also charged with armed robbery and kidnapping.
Early Wednesday, Stenson's body was found in a street in a residential neighborhood in Chandler, Arizona, a Phoenix suburb. Police say Stenson appeared to have been shot and then run over and dragged some distance by his own SUV.
Riddle, originally from Illinois, is the half brother of Kevin Riddle, 43, of Indiana, who was found driving Stenson's SUV about two hours after the body was found.
Police say Kevin Riddle has been charged with possession of stolen property.
In announcing the arrests late Thursday, authorities said they are still searching for another suspect. Anyone with information is asked to call police at (480)782-4130.
It's not clear at this point whether Stenson knew either of the Riddle brothers.
The East Valley Tribune reports the shooting is believed to have happened inside the SUV, at about 1:45 a.m. near Chandler Boulevard and Dobson Road. Witnesses told police that Stenson was pushed out of the vehicle and appeared to have been dragged by it as the killer or killers sped off.
Stenson, 25, made his major league debut this season and played 37 games for the Reds. He hit .247 with three home runs and 13 RBIs. His homer accounted for Cincinnati's final run of the season.
"I am terribly saddened by the sudden and tragic death of Dernell Stenson," said Baseball Commissioner Bud Selig. "My deepest sympathies go out to his family, his friends and his teammates."
The sad news spread shock through the Reds.
"It's an absolutely sick feeling," said hitting coach Ray Knight, in an interview with MLB.com, remembering Stenson as a guy who "didn't say much, but he had this smile that just grabbed your heart."
"I still have visions of him getting that big hit down the line in Philadelphia and that smile on his face," said first baseman Sean Casey, also on MLB.com. "That's the image I have of him - that smile on his face."
"I just can't believe it. what a waste," said Casey. "Who could do something like that? What is wrong with people?"
Stenson was playing this month for the Scottsdale Scorpions of the Arizona Fall League. He was hitting .394, third-best in the league, in 18 games.
Stenson was a third-round draft pick of the Boston Red Sox in 1996. He stayed in their system until this year, when the Reds claimed him off waivers during spring training.
Stenson played four seasons for Triple-A Pawtucket. In a statement Thursday, the team sent its condolences to the player's family, and said Stenson had been a "true gentlemen" during his stay in Pawtucket.