The club, whose fans were also involved in racist chanting during a match against Genoa in February 2015, were fined 50,000 euros ($55,980), Serie A's disciplinary tribunal said in a statement.
Play was held up for around three minutes in the second half after repeated booing was aimed at Napoli's Senegal international centre back Kalidou Koulibaly.
Koulibaly was subjected to "unequivocally racist chanting" from the stadium's Curva Nord "every time he touched the ball" in the second half, the statement quoted referee Massimiliano Irrati's match report as saying.
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Irrati said that the fourth official asked the public authorities at the stadium to halt play in the 62nd minute and make an announcement.
However, after no announcement was made in the next five minutes, the referee himself ordered the match to be suspended. Shortly afterwards, the announcement was made.
The tribunal imposed a one-match partial stadium closure covering the Curva Nord area of the Stadio Olimpico where Lazio's hardcore supporters gather. A similar ban which had been suspended since the Genoa match was invoked.
Lazio were fined a further 15,000 euros for "chants expressing of discrimination based on territorial origin" aimed at Napoli supporters.
Irrati only decided to continue the game after consulting with coaches and officials and Napoli secured a 2-0 win with goals from Gonzalo Higuain and Jose Callejon to stay top of Serie A.
In 2013, Lazio were ordered to play two European matches behind closed doors by UEFA following four different acts of racism by their fans.
($1 = 0.8932 euros)
(Writing by Brian Homewood in London; editing by Toby Davis)
