Veteran rock musician Bob Weir, the Grateful Dead’s rhythm guitarist who helped guide ‍the legendary jam band through decades of change and success, has died at age 78.

He was diagnosed with cancer in July and “succumbed to underlying lung issues” surrounded by loved ones, a statement posted to ⁠his verified Instagram account said.

“It is with profound sadness that we share the passing of Bobby Weir,” the statement said.

Grateful Dead founding member Bob Weir has died at the age of 78 after a cancer diagnosis in July. (AP PHOTO)

“He transitioned peacefully, surrounded by loved ones, after courageously beating cancer as only Bobby could. Unfortunately, he succumbed to underlying lung issues.”

Weir joined the Grateful Dead — originally the Warlocks — in 1965 in San Francisco at just 17 years old.

He’d spend the next 30 years playing on endless tours with the Grateful Dead alongside fellow singer and guitarist Jerry Garcia, who died in 1995.

Weir’s death leaves drummer Bill Kreutzmann as the only surviving original member. Founding bassist Phil Lesh died in 2024.

“For over sixty years, Bobby took to the road,” the Instagram statement said.

“A guitarist, vocalist, storyteller, and founding member of the Grateful Dead. Bobby will forever be a guiding force whose unique artistry reshaped American music.”

It was Weir who sang the verses ‌on the band’s trademark boogie anthem, Truckin’, and who wrote such key songs as Sugar Magnolia, Playing in the Band and Jack Straw.

The youthful, ponytailed “Bobby” ‍grew into an eclectic songwriter whose handsome appearance and diverse musical influences helped broaden the band’s appeal.

British newspaper The Independent called Weir “arguably rock’s greatest, if most eccentric, rhythm guitarist”.

After Garcia’s death at age 53, Weir carved out an interesting if somewhat neglected solo career – much of it with his band, RatDog – and participated in reunions of surviving Dead members in different configurations.