The partnership will focus on Cadbury’s central brand promise of ‘joy’ and will include content promoting seasonal trading and product launches.
On-site promotion will be integrated with user-generated native content, including videos and recipes, competitions and display advertising.
The chocolate brand will also sponsor the Mumsnet Classics forum and Mumsnet users will test and give feedback on Cadbury products and marketing.
In addition, Cadbury will sponsor Mumsnet Blogfest, a live festival for bloggers, in November.
Mondelez said the approach signalled the "continued shift to the digitisation" of its marketing.
"It gives us an exciting opportunity to speak to our core audience about our incredible brand heritage and portfolio of well-loved family product," said Matthew Williams, marketing activation director, Mondelez International.
Mumsnet has been ramping up its brand partnerships. Last month it struck a deal with Lidl to help position the discounter as "on mums' side".
However, the Children's Food Campaign expressed concern that the partnershp would give Cadbury "a clear route to marketing sugary snacks to families at a time when the focus should be on consuming less sugar."