Staff are dealing with an "internal incident" involving a male inmate overlooking the perimeter wall.
A prison service spokesman said: "A prisoner at HMP Cardiff has climbed an internal fence. Staff are working to ensure he comes down with no injuries."
A prison officer was standing on Adam Street monitoring the inmate at the jail in the Adamstown area of the city.
Four men were arrested in Cardiff prison in June after an explosive device "erupted".
Officers also had to deal with an "isolated incident" on Christmas Day 2016, a week after four prisoners barricaded themselves into a cell.

Chester's Ryan Lloyd cancelled out Kieran Murtagh's first-half opener, but Joey Jones gave Woking an interval lead and Chigozie Ugwu added a third in the second half.
Visiting goalkeeper Jon Worsnop was forced to push Woking right-back Jake Caprice's dangerous low cross on to the post in the ninth minute before Murtagh fired the home side ahead with a 25-yard drive into the bottom corner.
The visitors were back on level terms within eight minutes when midfielder Lloyd glanced home Johnny Hunt's cross, but Woking went in at the break 2-1 ahead as Max Kretzschmar's ball was headed in by Jones in the 28th minute.
Worsnop parried Kretzschmar's header and Woking substitute Zak Ansah fired into the side netting before the Cards went 3-1 ahead in the 75th minute through Ugwu's powerful drive.
Report supplied by the Press Association.
Match ends, Woking 3, Chester FC 1.
Second Half ends, Woking 3, Chester FC 1.
Gozie Ugwu (Woking) is shown the yellow card for a bad foul.
Substitution, Woking. Fabio Saraiva replaces Ismail Yakubu.
Substitution, Woking. Charlie Carter replaces Max Kretzschmar.
Goal!  Woking 3, Chester FC 1. Gozie Ugwu (Woking).
Zak Ansah (Woking) is shown the yellow card for a bad foul.
Substitution, Chester FC. James Akintunde replaces Jordan Chapell.
Substitution, Woking. Zak Ansah replaces Charlie Penny.
Evan Horwood (Chester FC) is shown the yellow card for a bad foul.
Jon Worsnop (Chester FC) is shown the yellow card for a bad foul.
Substitution, Chester FC. Elliott Durrell replaces Blaine Hudson.
Substitution, Chester FC. Sam Hughes replaces Johnny Hunt.
Second Half begins Woking 2, Chester FC 1.
First Half ends, Woking 2, Chester FC 1.
Max Kretzschmar (Woking) is shown the yellow card for a bad foul.
Goal!  Woking 2, Chester FC 1. Joey Jones (Woking).
Johnny Hunt (Chester FC) is shown the yellow card for a bad foul.
Goal!  Woking 1, Chester FC 1. Ryan Lloyd (Chester FC).
Goal!  Woking 1, Chester FC 0. Keiran Murtagh (Woking).
First Half begins.
Lineups are announced and players are warming up.
An "incident is ongoing" at Cardiff prison, the Ministry of Justice have confirmed.
[NEXT_CONCEPT]
Woking saw off Chester to secure their first National League win of the season.