One of the party's co-chiefs, Jörg Meuthen, and another leading member, Guido Reil, had received funds from Swiss advertising agency Goal AG in 2016 and 2017, respectively.
But German political parties are banned from receiving campaign funds from non-EU sources.
As a result, parliament's administrative body said it had imposed a fine of €402,900 , which essentially amounts to three times that of the illegal donations.
Both Meuthen and Reil are leading candidates for the AfD – Alternative for Germany – in May's European Parliament elections.
The far-right party faces further fines as one of its leaders, Alice Weidel, was also being investigated for having received around €132,000 from a Swiss source.
The six-year-old AfD, the country's biggest opposition party, opposes multiculturalism, Islam and the immigration policies of Chancellor Angela Merkel, whom it labels a “traitor”.
It shook up Germany's political landscape when it won 12.6 percent of the vote in the 2017 general elections, taking dozens of seats in the Bundestag for the first time.