The official announcement was made by their agent, after the pair attended a London film premiere together.
They announced a trial separation five weeks ago when actress Holden, 29, was pictured in a newspaper with Men Behaving Badly star Neil Morrissey - her co-star in TV drama Happy Birthday Shakespeare.
But she looked happy to be back with Family Fortunes host Dennis, 46, and the pair later went for a meal together.
Their agent, Pat Lakesmith, confirmed: "Amanda and Les are back together, but they do not want to talk about it, as it is very early days yet."
Adulterous
The statement came after the pair were seen at the screening of Another Life in the West End's Planet Hollywood restaurant.
The movie, which stars Lock, Stock and Two Smoking Barrels actor Nick Moran and Hornblower star Ioan Gruffudd, is about an adulterous woman wrongly accused of murdering her husband.
Afterwards, Holden showed she had not lost her sense of humour, telling reporters: "People being persecuted for adultery. I agree."
On her reunion with Dennis, she added: "It's early days, but things are looking good."
The couple were seen hugging and holding hands at the after-show party and later went off to have dinner at celebrity restaurant Zilli Fish with a female friend.