The campaign will run until April and aims to inform the public what the EU and the treaty means for the UK.
The treaty transfers some of the decision making powers from European Union institutions to the member states governments making the EU more democratic, said the FCO.
This means there will be more joint action between nation states on issues such as immigration, but states can maintain independence on issues such as tax.
Geronimo will manage a media relations campaign targeting publications that would not normally carry stories on European politics, such as regional press and broadcasting, trade press and women’s magazines.
An FCO spokeswoman said: ‘We want to target publications that traditionally have not covered these topics, which can be dry or technical.’
Geronimo will also manage a programme of ministerial visits around the UK led by European minister, Denis MacShane.
The agency won the £40,000 account after a four-way pitch against Grayling, Weber Shandwick and Harrison Cowley.
Each of the EU’s 25 members must ratify the constitution before it can take
A referendum is expected in the UK in spring 2006.