Westerwelle has been accused of taking with him on official trips businessmen close to him or his pro-business Free Democratic Party (FDP) or even his brother, who then use the occasions to secure deals.
There have also been eyebrows raised about the presence of the openly gay minister's partner, sporting event manager Michael Mronz, on a recent trip to Asia and one to Latin America last week.
"Such slander from the opposition towards the foreign minister while he is abroad to promote our employment, damages Germany," Westerwelle said in an interview in Bild newspaper late on Sunday.
"It is really the zero level of political culture when the opposition, for their own party interests, attack even the family of their political adversaries," Westerwelle said.
The minister also defended Mronz's presence during the last trip in the report.
"During the trip Michael Mronz visited several social establishments and gave private donations to projects that benefit children. He has not taken part in any political discussions but he went along to help children in the poor areas after the earthquake in Chile or in Brazil," he said.