The 28-year-old winger signed a one-year guaranteed contract with two option years, according to people familiar with the situation.
Najar was in training camp for all six weeks but did not appear in any of the five preseason friendlies because he needed to build up strength after years of injuries. Nonetheless, he impressed the technical staff in daily workouts.
If he had not reached a deal with United, he likely would have signed with an unspecified Dutch club, one person said.
Najar, a Honduran native who moved to Northern Virginia at age 13, rose through United’s youth academy and was the 2010 MLS rookie of the year.