The pair will appear on giant billboards and buses across Great Britain from this month, as well as in trade and women's press, as part of the Milk Marketing Forum's new £7.5m Make Mine Milk campaign.
The initiative aims to raise awareness of the health benefits of milk and highlight that low-fat milk contains less than 2% fat.
Online activity, social media and a wide range of PR will also be used. "The celebrities will be demonstrating that milk is today's cool and healthy dairy product on the block," said a campaign spokesman.
Lott and Ramsay follow in the footsteps of celebrities such as Andy Murray, Nell McAndrew, Amir Khan and Myleene Klass, who backed the Scottish White Stuff campaign between 2003 and 2006.
Scottish milk sales increased 3.4% between the end of 2003 the end of 2009, following the campaign.