Justin Timberlake & Jay Z -- This One's for Trayvon Martin

Justin Timberlake and Jay Z honored Trayvon Martin Friday night ... dedicating the song "Forever Young" to the 17-year-old shooting victim.

Jay and Justin kicked off their "Legends of the Summer" tour this week -- and during their closing number Yankee Stadium in NYC -Jay shouted to the audience, "Everybody put a cell phone and light it up ... let's light the sky for Trayvon Martin tonight in here."