Hilary Duff Brings Ex Joel Madden on Stage to Sing Good Charlotte's 'The Anthem'

Hilary Duff surprised fans at her sold-out concert Wednesday night ... bringing Good Charlotte to the stage ... yes, her ex Joel Madden's band!

The singer and actress thrilled fans when she welcomed Joel and his brother, Benji Madden, to the floor at her sold-out show at Madison Square Garden. She gave them a warm hug before they began performing the punk-rock band's hit song, "The Anthem."

Hilary actually dropped a hint about their appearance when she slipped on a Good Charlotte shirt during the show. She changes into a tee to rep an early 2000's musician at every concert ... even rocking her own 2004 tour shirt at sold-out shows last month at the Kia Forum in Inglewood.

The on-stage reunion was a major moment for fans who've followed Hilary from the very beginning ... she dated Joel from 2004 to 2006 ... and their 9-year age gap was controversial at the time because she was roughly 16 when they began dating, and he was 25.

The exes have been friendly over the years, though ... Hilary is close with Joel's wife, Nicole Richie ... and Hilary's husband, Matthew Koma, has collaborated with Good Charlotte.

They've posted about their gatherings over the years ... proving any drama from the early aughts is "So Yesterday."