Wales' largest and newest prison had an increase in violence last year - despite falls in incident rates at other jails across the country.
Prisoner attacks at Wrexham's HMP Berwyn jumped 143% to 561, according to a Cardiff University researcher.
Assaults on prison staff were also up a quarter to 257 - more than all other Welsh prisons combined.
The Ministry of Justice said prison violence across the UK as a whole was falling.
Report author Dr Robert Jones said it raised "further concerns over the performance of Wales' newest prison".
The £250m prison opened in February 2017 with the capacity to house 2,100 inmates.
By the end of 2019, it held 1,774 Category C prisoners - deemed the lowest-risk offenders in a closed prison.
Analysis of freedom of information requests, Ministry of Justice figures and Parliamentary questions revealed about 70% of the inmates came from outside of Wales.
The study found the massive jump in assaults by prisoners on other prisoners came as the jail's population rose by 18% over the year.
Not only does the prison have the highest increase in attacks, it also has the highest rate in Welsh institutions.
There were 39 incidents per 100 inmates at Berwyn in 2019, followed by 35 per 100 at Parc Prison in Bridgend, 25 in Cardiff and 20 in Swansea.
HMP Prescoed - an open prison - and Usk in Monmouthshire recorded just three incidents per 100 inmates.
The report also found HMP Berwyn had the highest number of hostage situations in the year, with 11 out of 14 incidents across Wales taking place there.
The prison topped the table for seized weapons - accounting for just under half of the 572 weapons found in all Welsh prisons.
"On average, there were five weapon discoveries a week at HMP Berwyn last year," said Dr Jones.
The number of weapons found at all Welsh prisons has been on the rise over the past three years - up a third in the year ending March 2020.
Despite the increase, the report noted the official prison inspections "made no mention of the scale of the problem in the Welsh prison estate".
Dr Jones's report also highlighted a worrying increase in the number of self-harm cases among prisoners in Wrexham.
Across Wales the trend for self-harm has been down - 2% over 2019 - with one notable exception.
Incidents of self-harm in Wrexham jumped from 551 to 1,015 in the space of a year - an 84% increase.
"At a time when the number of self-harm incidents, prisoner-on-prisoner assaults and assaults on staff have fallen across the Welsh prison estate, HMP Berwyn has bucked the trend with a rise in each area," added Dr Jones.
Andrew Neilson from the campaign group The Howard League for Penal Reform said: "Berwyn is a very troubled prison.
"This is clearly a prison that is under a lot of stress and strain, and that is showing in the violence figures and those self-harm figures.
"All of this shows that building these very large prisons is a mistake."
The Ministry of Justice said assaults in prisons had fallen by 8% in all UK jails for the year ending in March 2020.
An official added: "Tough new security measures are making it harder than ever to bring drugs into jails and this is one of the reasons assaults have fallen 8% in the past year."