Unidentified individuals threw the chemical in the former deputy prime minister's face on the street outside his campaign headquarters in Sochi.
The Russian opposition Solidarity movement nominated Nemtsov to run for mayor of the Black Sea resort city earlier this month. The city will host the Winter Olympics in 2014.
Elections for a new mayor will be held on April 26, and all nominations must be received by March 26.
The previous mayor, Vladimir Afanasyenkov, resigned on October 30, 2008, due to health problems.
As well as candidates from the governing United Russia party and the Communists, the main suspect in the Alexander Litvinenko murder case, Russian MP Andrei Lugovoi, may also be on the ballot.
Former Russian security officer Litvinenko died of radioactive poisoning in London on November 23, 2006. British investigators accused agent-turned-businessman Lugovoi of the murder, and demanded his extradition, sparking a major diplomatic row.
Moscow has refused to hand over Lugovoi, who has repeatedly denied involvement in the murder, citing lack of evidence and the Russian constitution, which does not allow extradition of Russian citizens to other countries.
The next Sochi mayor will oversee construction of infrastructure for the Winter Olympics in 2014 and most likely play a major role in the management of the sums involved. According to Prime Minister Vladimir Putin, the Russian government channeled 113 billion rubles ($3.25 billion) into the Olympics project in 2008, and will invest a further 127 billion rubles ($3.65 billion) in 2009.
