The deal includes an option for Barcelona to make the transfer permanent, with the fee reportedly set at around 30 million euros. Neither club has officially disclosed the financial terms.
Rashford, 27, arrived in Barcelona on Sunday and was officially unveiled before the team departs for their preseason tour of Asia later this week. He will wear the No. 14 shirt under new manager Hansi Flick.
"I'm very excited. This is a club where dreams come true," Rashford told Barca TV. "It feels like home already. It's a family club where people are comfortable and players can showcase their skills."
Barcelona's interest in Rashford dates back to January, and the move materialized after the club missed out on other left-wing targets. Nico Williams extended his contract with Athletic Club, and Liverpool rebuffed inquiries for Luis Diaz, prompting sporting director Deco to accelerate talks for Rashford.
Sources indicate Rashford accepted a significant salary cut to help push the deal through. Barca were also drawn to the structure of the agreement, which minimizes upfront financial risk amid ongoing budget constraints.
Flick was eager to integrate Rashford into the squad ahead of the tour, with friendly matches lined up against Vissel Kobe, Daegu, and FC Seoul - where Rashford could reunite with former United teammate Jesse Lingard.
Rashford's United career had stalled dramatically under new manager Ruben Amorim. After being dropped following United's Europa League clash with Viktoria Plzen in December, he was excluded from the squad entirely and spent recent weeks training away from the first team.
A January loan to Aston Villa failed to result in a permanent move, but Rashford - who has scored 138 goals in 426 appearances for United - was reportedly holding out for a transfer to Barcelona.
Though still under contract at United until 2028 on a 325,000 pounds-a-week deal, Rashford now looks to revive his form and reputation in Spain. He joins a Barcelona attack already featuring Lamine Yamal, Raphinha, Robert Lewandowski, Ferran Torres, and Dani Olmo, as Flick prepares for another push for European success.
Last season, the German led Barca to a domestic treble - LaLiga, Copa del Rey, and Supercopa - but fell short in the Champions League semifinals against Inter Milan.