Napoli, who is eligible to be a free agent at season's end, was traded along with cash considerations for a player to be named or cash considerations.
Napoli played with Texas in 2011-12.
"Once Nap cleared trade waivers, we reached back out to a handful of teams that we had been in touch with prior to the [non-waiver] Deadline and that all happened within the last 48 hours or so, and just happened that Texas ended up being a team that seemed like the best fit and an opportunity for him to go back to a place he's familiar with, and play in a role that should fit him," said Red Sox general manager Ben Cherington.
The trade of Napoli allows manager John Farrell to give more playing time to prospect Travis Shaw, who has performed well in limited opportunities.
Napoli, 33, is batting .207 this season with 13 home runs and 40 RBIs. In his last 14 games, however, the veteran has been batting .261 with a .370 on-base percentage and a .587 slugging percentage.
The Pirates were known to have shown interest in Napoli just days prior to the July 31 non-waiver Trade Deadline, but not much else surfaced at the time.
Napoli returns to the Rangers after spending three seasons with Boston, giving the Texas lineup a right-handed bat the team had been looking for, particularly as the lineup is predominantly left-handed. During his two seasons with the Rangers, Napoli was a .275 hitter, holding a .379 on-base percentage, and slugging .552.
Much like Shane Victorino, who was traded to the Angels in July, Napoli will always be remembered in Boston for his heroics in '13, and for being one of the unifying forces on a team that helped galvanize the city after the tragic Boston Marathon bombings.
"He was a core member of that team on the field, in the clubhouse, the unity in which that team was so strong in," Farrell said. "Nap was one of the leaders in that way. Middle of the order bat that allowed us to get off to such a great start with the production he had in April, and then in September and the postseason, he once again kicked it in."
It was somewhat fitting that Napoli was traded while the Red Sox were in Detroit. His solo homer in Game 3 of the American League Championship Series against Justin Verlander led the Red Sox to a 1-0 victory. Napoli added a mammoth homer to center against Anibal Sanchez in Game 5 of that series, which the Red Sox won in six games.
Victorino, Napoli, Jacoby Ellsbury, Jon Lester, John Lackey and Jonny Gomes are just some of the key figures from the '13 team that are no longer with the Red Sox.
"Yeah, it is part of the business, and unfortunately we've not been able to keep that group together for obvious reasons, and that's overall team performance and what the trading deadline signified," Farrell said. "That's the greatest disappointment in all of this, and then when you do get to a point, you're looking at who's [part of] the next phase, who's the next candidate to take some of those spots, and that's where we are."