The Irish musician, originally from Limerick, led the band to international success in the 90s with singles including Linger and Zombie.
A statement from her publicist said: "The lead singer with the Irish band The Cranberries was in London for a short recording session.
"No further details are available at this time."
A Metropolitan Police spokesperson said the police were called to a hotel in Park Lane at 09:05 GMT on Monday, where "a woman in her mid-40s" was pronounced dead at the scene.
The death is, at this stage, unexplained.
Her current band mates in The Cranberries - Noel Hogan, Fergal Lawler, and Mike Hogan - paid tribute to the lead singer on social media.
The message said: "She was an extraordinary talent and we feel very privileged to have been part of her life from 1989."
Her publicist added: "Family members are devastated to hear the news and have requested privacy at this very difficult time."
The parish priest from her home town Friarstown, in Limerick, said her funeral would take place in Ireland.
"Her family are still waiting for more details to come from London about her death," said Ballybricken and Bohermore parish priest Fr James Walton.
"The plan is for her to be buried here at home. When that will be will depend on when her body is released."
The Cranberries shot to international fame with their 1993 debut album Everybody Else Is Doing It, So Why Can't We? and went on to sell over 40 million records worldwide.
In 2017 The Cranberries announced a tour including dates in Europe, the UK, and the US.
However, in May - shortly into the European tour - the group had to cancel the remainder of the European dates as a result of O'Riordan's health issues.
The official Cranberries website cited "medical reasons associated with a back problem" preventing singer Dolores O'Riordan from performing.
But just before Christmas O'Riordan had posted on Facebook saying she was "feeling good" and had done her "first bit of gigging in months", leading fans to believe she would soon be performing again.