A three-judge Court of Appeal in Wellington said the Australian, 35, killed 51 Muslims at two Christchurch mosques in 2019 and filed his bid 505 days late.
Judges said prison conditions did not coerce his 2020 pleas and rejected claims of mental illness, calling the case meritless and saying he tried to mislead the court.
The ruling appears to end any chance of a trial, sparing survivors and bereaved families further trauma; Tarrant remains jailed for life without parole after dropping a separate sentence appeal.
With his appeal rejected, can the Christchurch killer ever use the courts again?
Can New Zealand manage its most dangerous prisoners without violating their human rights?
Why did the Christchurch attacker specifically want to be known as a terrorist?
Has New Zealand's fight against far-right extremism been successful since the 2019 attacks?
Is a new online safety regulator enough to prevent future livestreamed terrorism?
How are gaming platforms being regulated to stop the next white supremacist recruiter?