The deal, which includes two club option years, is pending a physical, which is why the team hasn't yet announced it.
In his first full season in the majors last year, Marte, a 25-year-old from the Dominican Republic, hit .280 with 12 homers, 35 RBIs and 10 triples in 135 games. He stole 41 bases but was caught a league-high 15 times, and he also struck out 138 times.
Marte, who reportedly turned down the team's first two offers, was scheduled to make $512,400 this season under the amateur free agent deal he signed in 2007. He wouldn't have been a free agent until 2018.
A natural center fielder, Marte moved to left because the Pirates already have NL MVP Andrew McCutchen in center.
McCutchen and Jose Tabata are also under club control until at least 2018, meaning the Pirates have their outfield intact for the foreseeable future.SI WIRE: Report: Dodgers have MLB's highest payroll at projected $235 million