| Steve Jones | 1988 (guest) | guitar solo on "Anarchy in the U.K."[48] | So Far, So Good... So What! (1988) |
| Jimmie Wood | 1994 | harmonica on "Train of Consequences" and "Elysian Fields" | Youthanasia (1994)[49] |
| Bob Findley | | - trumpet on "Silent Scorn"
- horn on "A House Divided"
| [50] |
| Heather Keckler | 2001 | spoken word on "The World Needs a Hero" and "1000 Times Goodbye" | The World Needs a Hero (2001)[50] |
| Suzie Katayama | 2001 | strings on "Promises" and "Losing My Senses" |
| Ken Mary | 2001 | Pro Tools |
| Chris Vrenna | 2001–2002 | [50] |
| Tim Akers | 2004 | keyboards | The System Has Failed (2004)[47] |
| Darien Bennet | additional vocals on "Blackmail the Universe" |
| Michael Davis | sound effects |
| Lance Dean | additional vocals |
| Scott Harrison |
| Celeste Amber Montague | additional vocals on "Blackmail the Universe" |
| Justis Mustaine | backing spoken word vocals |
| Ralph Patlan | - additional vocals
- spoken word
|
| Robert Venable | additional vocals |
| Jonathan Yudkin | |
| Eric Darken | | percussion | [51][52] |
| Charlie Judge | | - keyboards
- orchestral arrangements
| - The System Has Failed (2004)
- Dystopia (2016)
|
| Chris Rodriguez | | backing vocals | [47][53][54][55][52] |
| Axel Mackenrott | 2007 | keyboards | United Abominations (2007)[53] |
| Cristina Scabbia | backing vocals on "À Tout le Monde (Set Me Free)" |
| Brett Caldas-Lima | spoken word vocals on "United Abominations" |
| Marie Soler |
| Chris Clancy | 2009 | backing vocals | Endgame (2009)[54] |
| Mark Newby-Robson | keyboards on "The Hardest Part of Letting Go...Sealed with a Kiss" |
| David Draiman | 2013 | vocals on "Dance in the Rain" | Super Collider (2013)[56] |
| Yao Zhao | cello on "Dance in the Rain" |
| Tom Cunningham | - fiddle on "The Blackest Crow"
- violin on "Dance in the Rain"
|
| The Shannon Rovers Irish Pipe Band (Brian Costello, Sean Costello, Mary Kate Peterson) | bagpipes on "Built for War" |
| Electra Mustaine | backing vocals on "Forget to Remember" and "Beginning of Sorrow" |
| Sarah Phelps | backing vocals on "Beginning of Sorrow" |
| Willie Gee | guest speaker on "The Blackest Crow" |
| Farah Siraj | 2015 | vocals on "The Threat Is Real" and "Poisonous Shadows" | Dystopia (2016)[52] |
| Miles Doleac | voiceover in "Conquer or Die!" |
| Blair Masters | |
| Steve Di Giorgio | 2021 | bass | The Sick, the Dying... and the Dead! (2022)[51] |
| Ice-T | guest vocals on "Night Stalkers" |
| Sammy Hagar | guest vocals on "This Planet's on Fire" |
| Brandon Ray | additional vocals on "The Sick, the Dying... and the Dead!", "Life in Hell", "Sacrifice", "Junkie", "Killing Time", "Soldier On!", "Célebutante", "Mission to Mars" and "We'll Be Back" |
| Roger Lima | |
| Luliia Tikhomirova | voices on "Dogs of Chernobyl" |
| Bill Elliot | voices on "Junkie" |
| John Clement | voices on "Soldier On!" and "Mission to Mars" |
| The Marching Metal Bastards | voices on "Soldier On!" |
| Maila Kaarina Rantanen | voices on "Mission to Mars" |
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