NFL star Sam Darnold was crushed to learn his former USC teammate Chris Brown died over the weekend ... saying Monday the 24-year-old O-lineman was the "greatest dude EVER."

Brown passed away Sunday, according to Trojans football head coach Clay Helton ... who said he was "heartbroken" to relay the news to the SC community.

Darnold -- who was teammates with Brown for his entire USC career -- spoke out on the tragedy on his social media page Monday ... sharing three photos, and clearly reeling from the news.