Jackson, 28, who starred at Cal and was a first-round pick of Arizona in 2003, figures to get regular playing in left field, where he's seen most of his action this season.
The A's were swept by the Giants over the weekend, a series that exposed their need for help at the plate. Jackson, a right-handed hitter, was hitting just .238 with one homer and 11 RBIs in 42 games with Arizona. He missed more than two weeks early in the season with a strained right hamstring and played in just 30 games last year after coming down with Valley fever, a fungal infection.
He's a career .277 hitter and has 46 homers over five-plus big league seasons. His best season to date was in 2006, when he hit .291 with 15 homers and 79 RBIs. His .358 career on-base percentage particularly caught the A's attention.
"We've been looking for a right-handed bat to fit in this lineup and lengthen it out," assistant general manager David Forst said. "We've gone through series where we've swung the bat really well and periods where we've had trouble scoring runs.
"It's always going to be this way with this team until we get all our guys healthy. This hopefully is a boost for our offense to get us over the hump in some games."
The move also makes sense to Jackson.
"I think a change of scenery might be a good thing," Jackson told the Arizona Republic. "I had a great time there with all the guys. That's like my second family. I came up with all those guys, created unbelievable relationships with those guys in there. I've known them for so long. It's going to be a little different."
The A's, 32-33 and four games behind first-place Texas in the AL West, hope to get outfielder Coco Crisp back from a rib-cage injury within the next week. But their lineup needs power more than anything.
Jackson is a natural fit in left field, but with the A's in the middle of a nine-game stretch in National League parks and no DH available, Jackson may split time with Jack Cust in left for the upcoming games.
The A's open a three-game series against the Chicago Cubs tonight and then play three at St. Louis. It's unknown if Jackson will arrive at Wrigley Field in time for tonight's game.
How well the A's play over the next few weeks figures to influence whether they acquire more help before the July 31 non-waiver trade deadline.
"We'll see how things go over the next month-and-a half," Forst said. " If the right fit is available, we'll certainly stay open-minded to help this team out."
Demel, a right-hander, was a third-round pick of the A's in 2007. He was 2-0 with six saves and a 1.26 ERA this season with Triple-A Sacramento.
Jackson is on a one-year, $3.1 million deal for this season. The A's received $400,000 from Arizona to help offset his salary. He'll be arbitration-eligible again after the season and can hit free agency for the first time following the 2011 season.