The man who allegedly shot David Ortiz has been arrested ... officials announced Wednesday -- saying the shooter, along with 5 other suspects, were paid 400,000 pesos to kill Big Papi.
As we previously reported, Ortiz was shot in the back on Sunday night as he was sitting in an outside patio area of a popular Dominican Republic nightclub.
D.R. cops say two young men drove up to the scene on a motorcycle ... when one of them got off to fire a single bullet into the Red Sox legend's lower back.