Armstrong's first guitar was a cherry-red Hohner acoustic which he received from his dad, which he mentions in the book Nobody Likes You: Inside the Turbulent Life, Times and Music of Green Day. Armstrong received his first electric guitar, a Fernandes Stratocaster copy that he named "Blue", when he was eleven. Armstrong said he was "shocked" when his mother bought the guitar from Armstrong's teacher, because he knew their family could not readily afford it. He modified the guitar to have a humbucking pickup in the bridge position. He initially installed a Bill Lawrence pickup but later switched to the Duncan JB model. He toured with this guitar from the band's early days and into their first few major-label tours. "Blue" also appears in several of their music videos starting with "Longview","Basket Case", and appearing most recently in "Minority". Armstrong has made several replicas of this guitar and uses them at concerts. He also owns several Gibson SG models which he has removed all of the sound/tone knobs except for one volume.

Today Armstrong mainly uses Gibson and Fender guitars. Twenty of his Gibson guitars are Les Paul Junior models from the mid- to late-1950s. His Fender collection includes: Stratocaster, Jazzmaster, Telecaster, and his Fernandes, "Blue". He states that his favorite guitar is a 1956 Gibson Les Paul Junior he calls "Floyd". He bought this guitar in 2000 just before recording their album Warning.

Armstrong also has his own line of Les Paul Junior guitars from Gibson.

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