The altercation played out during a daily morning political talk show.
The spokesman threw water at a left-wing female politician, and then smacked another three times across the face.
Prosecutors immediately issued an arrest warrant for the man, identified as Ilias Kasidiaris, whose party alarmed Europe by gaining 21 of Parliament's 300 seats in Greece's inconclusive May 6 elections.
His party, called the Golden Dawn party, has been accused of leading violent attacks on immigrants.
Golden Dawn, which vehemently denies the neo-Nazi label given by some opponents, has denied involvement in the attacks, insisting it is a nationalist patriotic group campaigning on a platform of ridding the country of illegal immigrants and cleaning up crime-ridden neighborhoods.
All of this leads up to elections being held next week.
The Associated Press contributed to this report.