The band made the announcement on Friday.
"As most of you probably know, our brother James has been struggling with addiction on and off for many years. He has now, unfortunately, had to re-enter a treatment program to work on his recovery again," a message from the band read, reports variety.com.
The tour was scheduled to begin in Perth, Australia, on October 17 and end on November 2 in Auckland, New Zealand.
Drummer Lars Ulrich, lead guitarist Kirk Hammett and bassist Robert Trujillo said the band intends to travel to the planned tour locations "as soon as health and schedule permit."
While the nature of his addiction was not revealed, Hetfield has undergone treatment for alcoholism over the years, as detailed in the band's 2004 documentary "Some Kind of Monster."
While the tour was postponed and not cancelled, the group offered to refund all tickets for the effected leg of the tour. As of Saturday, the 2020 dates, which include a March benefit show in San Francisco and an April tour of South America, were still on.
"Once again, we are devastated that we have inconvenienced so many of you, especially our most loyal fans who travel great distances to experience our shows," the statement said.
"We appreciate your understanding and support for James & thank you for being a part of our Metallica family."
Metallica was formed in 1981. The band's fast tempos, instrumentals and aggressive musichip made them one of the founding "big four" bands of thrash metal, alongside Megadeth, Anthrax and Slayer.
They are known for songs like "Nothing Else Matters", "Master of Puppets", "Fade To Black" and "Whiskey in the Jar".
( With inputs from IANS )