Interactive Investor, publisher of titles such as Money Observer and Moneywise, specialises in providing financial data and news. Meanwhile, CityAM has made its mark as the capital's free business newspaper.
The tie-up between the two will involve sharing of editorial and data resources. Combined editorial content, for instance, will be available on CityAM's website.
CityAM claims the combining of 36 journalists will give "the alliance editorial clout to rival any news-gathering organisation and will reach hundreds of thousands of people across the UK each day."
Complementary skills between the two include economics, banking, investment, politics and personal finance.
Lawson Muncaster, CityAM's managing director, said: "Combining CityAM's journalism with the power of data and journalism provided by Interactive Investor creates a rich resource for business and financial audiences.
"We believe this alliance is well positioned to provide the vital information of which the City makes and shapes its decisions."