The Doctor Who star confirmed he had a new baby daughter, adding he was chuffed the couple now have "two of each".
"Number four has arrived," Tennant said during his co-hosting stint on Absolute Radio when asked about his "growing brood".
"It's getting a bit... it's a bit of a brood yeah," he added.
Tennant is already dad to Olive, four, Wilfred, two and son Ty, 13, who the actor adopted when he married Moffett.
The Broadchurch star didn't reveal any further details about the couple's newborn daughter, keeping quiet on her name and when she was born.
With a larger family of six, Tennant was quizzed about what Christmas will be like in the household this year by fellow co-host Christian O'Connell.
"It certainly will be busy, a sleepless Christmas I suspect," said Tennant.
The couple met in 2008 when Moffett had a guest role on Doctor Who, and they married on 30 December 2011.
Moffett became a mum aged 17 to her son Tyler, whom Tennant adopted not long after Moffett gave birth to Olive.
At the time, he said according to the Mail Online: "My baby is almost a year old and I adopted my wife's boy last year as well, so I became a father twice in six months - that's got to be something of a record hasn't it?"