Kelly posted on Facebook that he and Giffords support Barber and urged donations to his campaign.
"Ron is a leader who puts politics aside and brings people together," Kelly also wrote in an e-mail seeking donations for Barber.
Because of goodwill in Tucson for Giffords and Barber after the shooting, the joint Giffords-Kelly endorsement stands to help Democrats keep the seat, said William Dixon, a University of Arizona political science professor.
The special election to fill her Eighth District seat is set for the spring. Barber said he had not decided whether to run for a full term in the regular election set for November in the redrawn Second District.
"I'm not a politician, never run for office before, so I'm taking this one step at a time," Barber said.
Barber and Giffords were among 13 people wounded in the Jan. 8, 2011, attack outside a Tucson supermarket. Six others were killed, including a 9-year-old girl.
A month later, Barber said he was setting up a nonprofit to fund school programs and community events.
The special election primary is set for April 17 and the general election June 12.
One other Democrat, State Rep. Matt Heinz, had announced his run for the seat. But Heinz said Thursday that he would respect Giffords' support for Barber in the special election and now run only for a full term. "I will refocus my efforts for the fall," Heinz said.
Five Republicans are in the race.