The charity is running banners and sponsoring editorial on the Delia Online homepage, to include various themed recipe galleries during September.
Users can click through to an event microsite where they are encouraged to organise their own coffee morning on September 30.
The advertising will be supported by email newsletters sent to Delia Online's 125,000 subscribers. Advertising agency PHD and vcmedia negotiated the campaign. Creative was developed by Ideas Eurobrand.
Helen Gray, national events manager at Macmillan Cancer Relief, said: "We're really excited about our first online advertising campaign. The reach and number of users that deliaonline.co.uk has will enable us to talk to almost 400,000 users. Many of those users are our target demographic for World's Biggest Coffee Morning".
Vanessa Kent, UK country manager at vcmedia, said: "Macmillan's campaign is a great fit for users of Delia Online."
She added: "Advertisers can reach very targeted audiences and meet their individual campaign objectives."
The World's Biggest Coffee Morning is in its 15th year and will be supported by press and radio advertising.
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