Who is the first character to say 'Avengers, assemble!' in the MCU?

Captain America is the first character to say “Avengers, assemble!” in the MCU. Steve Rogers delivers the complete line during the climactic Battle of Earth in Avengers: Endgame (2019), after Doctor Strange opens portals that bring the returned Avengers and their allies onto the battlefield against Thanos.

The moment had been anticipated for years because “Avengers, assemble!” is one of the team’s best-known comic-book rallying cries. The full phrase was deliberately absent from The Avengers (2012). In Avengers: Age of Ultron, Captain America says only “Avengers...” before the scene cuts away, teasing the line without completing it.

There is an important distinction between the MCU and the comics. In Marvel Comics, Thor was the first character to say the phrase, in Avengers #10 in 1964. Captain America later became the character most strongly associated with it. Therefore, Captain America is the correct answer when the question specifically asks about the MCU, while Thor is the comic-book answer.