The German Football Association (DFB) has "categorically rejected" allegations of a secret fund to secure votes to host the 2006 World Cup.
German news weekly Der Spiegel says a 6.7m euro (£4.9m) fund was set up using private money from the late former Adidas boss Robert Louis-Dreyfus.
Fifa said in a statement the "serious allegations" would be investigated while the DFB plans legal action.
Germany won the vote by 12 votes to 11, narrowly ahead of South Africa.
"The DFB categorically rejects the baseless allegations by the publication that the association had paid illicit funds in relation to the 2006 World Cup," the German FA said in a statement.
"The association also rejects the completely unsubstantiated conclusions that these funds were used to buy votes in exchange for the World Cup."
Fifa said in a statement: "These are very serious allegations that will be reviewed as part of the independent internal investigation currently being conducted by Fifa under the direction of its legal director with the assistance of outside counsel."
Der Spiegel claims the money was later repaid to Swiss businessman Dreyfus in 2005 using Fifa as cover - the German World Cup organising committee made a 6.7m euro contribution for a gala opening ceremony at Berlin's Olympic Stadium, which was later cancelled.
The publication alleges the money was then paid into a Fifa bank account in Geneva and from there transferred to a Zurich account belonging to Louis-Dreyfus, who has since died.
Der Spiegel also claims 2006 World Cup organising committee president Franz Beckenbauer and German FA president Wolfgang Niersbach were also aware of the fund by 2005, which the German FA denies.
It states neither Beckenbauer nor Niersbach "were involved in such transactions, or indeed could have had any knowledge of them" and added the governing body would "be taking legal action against the claims made by the magazine Der Spiegel."
Beckenbauer and Niersbach have not personally responded to the allegations.