The Red Sox will receive a player to be named later or cash in the trade.
Napoli played two seasons with the Rangers, in 2011 and '12, before joining the Red Sox as a free agent in the ensuing offseason.
Rangers general manager Jon Daniels said reacquiring Napoli fills the team's need for a right-handed bat with power.
"He is a guy who always hits lefties. It's an area where we have struggled this year," Daniels said on a conference call Friday night. "He is a guy we know. We know he's going to fit our culture. He has a winning makeup."
While in Texas, Napoli batted .320 with a career-high 30 homers as the Rangers won their second consecutive American League pennant in 2011. He hit .350 with two home runs and 10 RBIs in the World Series loss to St. Louis.
The following season, Napoli had 24 homers in 108 games and was the AL's starting catcher in the All-Star Game.
After signing with the Red Sox, he played an integral role in the team's 2013 World Series championship.
When a chronic condition nullified his original three-year deal with Boston, making him a free agent again after the 2013 season, Napoli rejected a strong bid from the Rangers to return to Texas.
Instead, he signed a two-year, $32 million deal with Boston, which he had embraced as his offseason home. That contract was set to expire at the end of this season. With the Red Sox out of contention and the Rangers trying to climb back into the AL West race, the Red Sox were open to dealing Napoli, who turns 34 on Oct. 31.
"Once Nap cleared trade waivers, we reached out to a handful of teams we at least had dialogue with prior to the deadline," Red Sox general manager Ben Cherington said. "This all happened in the last 48 hours or so; Texas seemed like the best fit.
"I talked to Nap. He was raring to go back to a place he's familiar with and play a role that should suit him and play for a team that's made a nice run and could do something down the stretch. I'm happy for Nap and obviously want to thank him for what he did for the Red Sox, certainly, and particularly in 2013 -- he was a huge part of our success that year."
With his return to Texas, Napoli is back in the playoff chase. The Rangers (54-54) had won four straight before losing Friday night's series opener inSeattle4-3, and are 3 games behind in the race for the second AL wild card. Texas is 5 back of first-placeHoustonin theAL West.
"This is really about 2015. He fills a need for us," Daniels said.
Napoli is expected to join the club Saturday, when the Rangers are set to face Seattle lefty Mike Montgomery. Daniels said the initial plan is for Napoli to be the DH or first baseman against left-handers, and only be used as an emergency third catcher, similar to the role he has had with the Red Sox.
Texas transferred Kyle Blanks to the 60-day disabled list to clear a spot on the 40-man roster. The Rangers will need to make another move to open a spot on the 25-man roster when Napoli arrives.
Napoli's production slipped last season, when he dealt with toe, finger, back and knee injuries that limited him to 119 games. He hit 17 home runs, his fewest since he became an everyday player in 2009, and drove in 55 runs while posting a slash line of .248/.370/.419/.789.
During the offseason, he underwent major surgery that involved the fracturing of his upper and lower jaw to alleviate a condition known as obstructive sleep apnea, which he said had deprived him of a good night's sleep for eight years. It had gotten so bad, according to Napoli, that he contemplated retirement.
The surgery was a success, and Napoli says he's resting better than he has in years. He also had offseason toe injury.
Napoli has struggled mightily at the plate this season. On July 22, he was batting only .198. But in 14 games since, he's posted a .957 OPS with six doubles and three home runs, raising his overall average to .207.
Napoli was scheduled to start Friday night's game for Boston at first base until the team announced a lineup change just before game time.
Cherington made it clear this week that the team's preference is to keep Pablo Sandoval at third base and Hanley Ramirez in left field. Travis Shaw played first base in Friday's 7-2 win over theDetroit Tigers, and the Red Sox could recall right-handed-hitting Allen Craig, the formerCardinal, from Triple-A Pawtucket.
Craig has posted a .274/.379/.341/.720 slash line at Pawtucket, but with just 11 extra-base hits (8 doubles, 3 home runs) in 252 at-bats. He was batting .324 in his past 10 games.
Information from The Associated Press was used in this report.
Napoli heading back to Texas
ESPNBoston.com's Gordon Edes discusses why the Rangers acquired Mike Napoli from the Red Sox and what Boston's motivation is in letting go of the first baseman.