The "Aquaman" star joined thousands Wednesday on their 17th day of blocking a road that leads construction crews to Mauna Kea's summit ... where crews plan to build a 30-meter telescope. Jason was flanked by his children ... son, Nakoa-Wolf Momoa, and daughter, Lola Momoa, both of whom were wrapped in native flags.

Jason -- who was born in Hawaii -- wore a crown and green leaf lei around his neck. He told the crowd he was honored to be there and followed with, "We are not going anywhere," which drew a roaring ovation.

As we reported ... Dwayne "The Rock" Johnson joined the group last week and said, "It's bigger than a telescope. It's humanity. It's culture." Native Hawaiian groups, of course, are worried the construction site will damage land they consider sacred. The astronomers behind the Thirty Meter Telescope want to put it on Mauna Kea's summit because it has the best conditions in the Northern Hemisphere.

Damian Marley was also there over the weekend ... singing his famous dad's classics while reminding the masses they're doing important work.