Organisers Run 4 Wales announced the news after 22,000 runners took part in this year's Cardiff Half Marathon.
Chief executive Matt Newman said there was massive demand for the full 26-mile event in Wales and details would be announced in the next two weeks.
"People keep asking me about a marathon so we are going to put one on," said Newman.
"Run 4 Wales will be putting on a full marathon in April as a precursor to the London Marathon.
"There is huge demand. Plans are in place and almost finalised and we should be ready to launch in the next two weeks."
This year's Cardiff Half Marathon proved a huge success with organisers saying a record field had taken part.
In perfect conditions, course records were broken in both the men's and women's races.
Cardiff is to host a full marathon in April 2017.