The Big Sleep Hotel, once part-owned by Hollywood star John Malkovich, received complaints from guests about blood stains on walls, rubbish on floors and unclean bathrooms.
X-Ray also found dirty duvets, mouldy windows and dead insects.
The hotel, now owned by Compass Hospitality, has promised to improve.
The Big Sleep, opposite the Motorpoint Arena, was hailed as a "designer budget hotel" when it was opened by Malkovich in 1999.
But unhappy customers have spoken to X-Ray about how shocked they were by the cleanliness in the rooms when they visited.
Jan Jones, 58, from Porthcawl in Bridgend county, paid Â£68 to stay at the hotel following a night out with friends in January.
But she said when she went to her room, it was strewn with rubbish and the bed was unmade.
"I really thought it was occupied by someone because it was just an absolute mess," she said.
"There were empty food containers in there, there were empty beer bottles, the mattress was exposed and it was stained.
"The bathroom was filthy dirty. And the smell was awful because there was stale food in there. It was just shocking."
Another guest, Lina Tsaltampasi from Thessaloniki, Greece, who stayed in September, bought cleaning products to clean the room herself.
After speaking to guests, the BBC X-Ray team also booked a room at the hotel and presenter Omar Hamdi found a broken bed and a stained mattress and bedding.
"Everywhere I looked there was grime," he said.
"A blood stain on the wall, hairs on the bedding, thick dust on the skirting boards, and stains in not one, but both drawers of the bedside table."
The Big Sleep apologised and said its "lapses in quality" were unacceptable.
It said it was taking action to improve its housekeeping and to tackle poor management.
It added it was confident the hotel would be transformed once its current refurbishment was complete.
The hotel has said it will refund all the customers featured on X-Ray.
A hotel in the heart of Cardiff has admitted "unacceptable" failings, after a BBC Wales investigation uncovered dirty rooms.