Dennis, 56, who played history teacher Pete Brockman in the hit BBC sitcom, says he and Skinner, 53, his on-screen wife Sue, have been together for more than a year.
The relationship blossomed after both split up with their previous partners in quick succession, Dennis told the Mail on Sunday.
“I am very, very happy, we are so very happy,” he said: “It’s nice and yes, it’s so lovely.”
Dennis had been married to second wife, Kate Abbot-Anderson, which whom he has two children, for 19 years before they separated in 2016.
Skinner is thought to have split from her former husband and the father of her two children, director Charles Palmer, in 2015.
A friend of the couple also told the Mail the pair were “very much in love”, but had chosen not to “go public” with their relationship until this point.
“It’s the most wonderful love story, they found one another at the right time in their lives and it’s all rather sweet.
“Over the past few weeks there has been some talk within the acting world of them being an item.”
Skinner and Dennis played parents to three precocious young children, Jake, Ben and Karen, played by Tyger Drew-Honey, Daniel Roche and Ramona Marquez, in the popular sitcom, which aired on BBC One for five series between 2007 and 2014.
In September last year, Dennis said Outnumbered could return for one-off episodes or even a new series after the success of its 2016 Christmas special.
He added the fact the actors playing the show’s three children were now in their late teens and early twenties did not matter because: “kids tend to live with their parents now until they’re about 40.”