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.

The agency says more than 700 divers have visited the site, 4,000m under water off the coast of Canada.
The ship will fall under the 2001 Convention on the Protection of Underwater Cultural Heritage once it passes the 100th anniversary of its sinking on 15 April.
The convention aims to prevent unscientific or unethical exploration.
Irina Bokova, director-general of Unesco, said the sinking of the Titanic was "anchored in the memory of humanity" and it was important to protect the site where 1,500 people lost their lives.
"There are thousands of other shipwrecks that need safeguarding as well... We do not tolerate the plundering of cultural sites on land, and the same should be true for our sunken heritage," she added.
The 2001 convention only protects vessels that sank more than a century ago.
The Titanic, built in Belfast, sank on its maiden voyage after setting off from the British port of Southampton.
It hit an iceberg off the coast of Newfoundland in Canada en route to New York, sinking two-and-a-half hours later.
A hotel in the heart of Cardiff has admitted "unacceptable" failings, after a BBC Wales investigation uncovered dirty rooms.
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The wreck of the Titanic is to come under the protection of the United Nations cultural agency Unesco.