The PR firm has been hired to improve the foreign image Ennahda, which was inspired by Egypt’s Muslim Brotherhood.
According to filings with the US State Department, Burson-Marsteller will undertake certain political activities on behalf of the party.
It will "arrange meetings between Ennahda representatives and stakeholders, and encourage support for free and fair elections in Tunisia" via the party’s offices in London.
The registration statement also states that Burson-Marsteller will "provide Ennahda support on media and stakeholder outreach in advance of upcoming elections" with "fees and expenses to be decided at a later date".
A parliamentary election is due to be held in Tunisia on 26 October, with presidential elections held nearly a month later.
A spokeswoman for Burson-Marsteller said: "Burson-Marsteller does not represent the Muslim Brotherhood. We represent the Ennahda (Renaissance) party. The party, which won the first democratic elections in Tunisia, helped to establish Tunisia’s constitution, which is widely regarded as one of the most progressive in the Arab world."