William M. Adler (born January 26, 1976) is an American guitarist in the groove and thrash metal band Lamb of God. William is the younger brother of drummer Chris Adler. Adler joined Lamb of God after its lead guitarist, Abe Spear, left the band.

Willie Adler taught himself how to play the guitar and is known for his unconventional playing style, including dissonant chord shapes, unique timings, distinct rhythms, extensive use of his left pinky finger, wide spreads between the frets and a tight hitting hand. He mainly plays the sharp staccato riffs in Lamb of God songs, while Mark Morton plays the groovy rhythms and solos.

Adler is one of the main writers in Lamb of God. Some of the songs he has written are « Black Label », « 11th Hour », « Blood Junkie », « Hourglass », « Blood of the Scribe », « Beating on Death’s Door », « Ashes of the Wake », « Ruin and « Again We Rise ». Adler also co-writes songs with Mark Morton, such as the pre-solo breakdown of « Walk With Me in Hell », and the entire track « In Your Words » to the end written by Mark Morton. Adler worked with metal band Vehement (Houston) in 2008 on their six-track promo CD.