The cost of operating the city’s jails has hit a record $1.36 billion — even as the inmate population reached a low of 9,500, Comptroller Scott Stringer said in a study released Tuesday.

That’s pushed the average cost of housing each inmate to $143,130 a year — an 8.5 percent increase between fiscal 2016 and 2017.

When pension and other expenses are included, the average cost per inmate was $270,876.

“An extraordinary decline in inmates should yield cost-savings and better all-around outcomes – not dramatic spending increases. That’s what’s so alarming about the numbers,” said Stringer.

His analysis also showed showed increases in fights and assaults among inmates, as well as attacks on staff.

City Hall spokeswoman Natalie Grybauskas defended the mayor’s reforms of the troubled jails system.

“We’re proud of our success in reducing the jail population, and we’re proud of the reforms that have made Rikers safer for staff and inmates,” she said.

“Our investments in safety and skills development for staff and inmates cost money but have been key in improving conditions in our jails.”