The deal will create a business with combined net revenues in 2014 of €1.3bn ($1.4bn, £950m)
It says the new group will attract more than two million high-spending customers and some 24 million visitors to its sites a month.
Swiss luxury group Richemont, which owns Net-a-Porter, will hold on to 50 per cent of the shares.
But it will be restricted to only 25 per cent of the voting rights in the new Yoox Net-A-Porter Group.
Federico Marchetti, founder of Yoox, will be chief executive of the new company.