The term "negotiations" was added to the previous title of this portfolio, as Juncker has stated there won't be any enlargement during his five-year mandate.
Serbia, Montenegro and Turkey are currently in accession negotiations and Macedonia and Albania are also considered as candidate countries.
Hahn's most challenging task however will be dealing with the situation in Ukraine. The current conflict was sparked when former president Yanukovych's cabinet abandoned an agreement on closer trade ties with EU, instead seeking closer co-operation with Russia.
Hahn, 56, previously served as the European commissioner for regional policy under outgoing European Commission President Jose Manuel Barroso.
French former Finance Minister Pierre Moscovici, a socialist, gets the key EU economic and monetary policy portfolio in the new 28-strong commission.
British politician Jonathon Hill, a former leader of the House of Lords, will oversee financial stability and financial services.
Germany's Guenther Oettinger will have responsibility for the digital economy, a portfolio that includes the overhaul of the EU telecoms market.
Denmark's Margrethe Vestager will be the commissioner for transport/competition and former Slovenian Prime Minister Alenka Bratusek is to oversee EU efforts to create an energy union.
Overall, nine of the posts went to women.
Juncker, who served for many years as Luxembourg’s premier, said his choices were aimed at "moving away from static structures."
The Commission is seen as the most powerful EU institution, as it drafts EU laws, ensures compliance with EU treaties and negotiates far-reaching trade deals with international partners.
Juncker said seven vice presidents would help coordinate the work of the new commission, on which each of the 28 member states has one seat.
The new Commission should take office in early November.