On 10 November 1986 a label that ran out of a record shop in Belo Horizonte pressed an album by four teenagers, taped that August at Estudio Vice Versa, self produced because nobody else was going to do it, thirty three minutes long and credited to a bassist who by several accounts did not play a note on it. That album is Morbid Visions. It is a death metal record. It is also the second to last one Sepultura would make.
So the answer to the question is yes and no, and unlike most yes and no answers this one has exact dates on both sides.
The part that is death metal

Sepultura begin as a death metal band, and there is no honest way around it. The first release is Bestial Devastation, a side of a split with Overdose issued by Cogumelo in December 1985, made when Igor Cavalera was fifteen. Then Morbid Visions. Both records take what Venom and Celtic Frost were doing, run it faster than the players could control, and put a voice on top that is nowhere near singing. The tempo is death metal tempo. The vocal is a growl. The subject matter is exactly what the Florida bands were writing about in the same eighteen months.
They are also poor records, and the affection they attract now is mostly retrospective. The playing is loose, the drums are recorded badly and the guitars have almost no bottom to them. That is not a criticism aimed at anybody: four teenagers in a country where an imported album cost most of a week's wages, with second hand gear and a label operating out of a shop, made the record they could afford to make. It sounds like it. It is still death metal.
The part that is not
The turn happens in 1987, faster than most bands manage in a decade. Jairo Guedz left, Andreas Kisser came in on lead guitar, and Schizophrenia arrived in October 1987 as a thrash album: cleaner, tighter, riffs written in figures rather than in noise, and a rhythm section suddenly capable of playing what was written. Two years later Beneath the Remains came out on Roadrunner, and from that point Sepultura are one of the best thrash bands in the world and are not a death metal band in any useful sense.

Arise, released on 25 March 1991 and recorded at Morrisound Recording in Tampa with Scott Burns on a Roadrunner budget of around forty thousand dollars, is the record that confuses people most, and it should not. It reached 119 on the Billboard 200. It was made in the same room as Cause of Death and Effigy of the Forgotten, by the same engineer, with the same dry gated guitar sound, and it still is not death metal, because a studio is not a genre. The riffs are locked sixteenth note thrash figures with the picking hand doing thrash work. Max Cavalera's voice is a shout with pitch in it. There is not a blast beat on the album.
Same studio, same engineer, same year, same country of origin as half the genre. Still thrash. A room is not a genre.
After that the distance only grows. Chaos A.D. in autumn 1993, made at Rockfield in south Wales with Andy Wallace, slows everything down and opens the arrangements out. Roots in 1996 goes further again. Whatever those two records are, and they are both good, nobody has ever seriously proposed them as death metal.
What the reference works do with it
The encyclopaedias handle this the sensible way, by refusing to pick one label for a forty year career. Sepultura are filed as death metal for the early period, thrash and groove for the middle, and a hyphenated mess for the later stretch, and that is the correct answer written out in full. A band is not obliged to be one thing, and the ones that stay one thing for forty years are the exception rather than the rule.
The band themselves have never argued about it either. They were a death metal band because that is what was arriving in Brazil by post in 1985, and they stopped being one when Kisser walked in with a different technique and the songs changed to fit him. There was no announcement and no disavowal. The records just changed.
There is a late footnote worth having. The Derrick Green era, which has now lasted longer than the Max Cavalera one, keeps going back to the harder end: Dante XXI in 2006 and Quadra in 2020 both carry stretches that are faster and more extreme than anything the band recorded in the middle of the nineties. It is still not a return to Morbid Visions, and it was never meant to be. It is a thrash band using the whole width of what it can play, thirty years after it worked out what that was.
Why the question keeps coming back
Three things keep it alive, and all three are worth naming. The first is Morrisound: for a stretch around 1990 the studio was a death metal factory, and a band that walked into it acquired the accent whatever they were playing. The second is the subject matter of the early sleeves, which is indistinguishable from what Florida was printing. The third is the company they kept in Belo Horizonte, an underground of bands whose records genuinely were extreme in the way this genre means.
None of those is a musical argument. Genre is decided by what the hands are doing: the tuning, the picking pattern, the beat under it and the placement of the voice. Run those four tests on Morbid Visions and you get death metal. Run them on Arise and you get thrash, four times out of four. If you want the full set of bands where all four answers come back the other way, that is our death metal band list.
The answer has a date on it
Sepultura are a thrash band who started out playing death metal and stopped in 1987. Both halves of that sentence are true and neither cancels the other, and anyone who tells you the band were never death metal has not heard the first two records.
It is now an argument with a closing date. The band announced in December 2023 that they were finishing, the farewell tour opened at the Arena Hall in Belo Horizonte on 1 March 2024, and the last show is booked for 7 November 2026 at the Mercado Livre Arena Pacaembu in Sao Paulo, forty two years after they started. Jairo Guedz, who played the guitar on both of the records this article says are death metal, is announced as a guest. The genre gets the last word after all. The same question asked of a much younger French band with a comparable career arc is answered in is Gojira death metal.