The lawmakers announced that the March 11 adoption of the declaration is a technical step ahead of a referendum on March 16 that will ask voters whether Crimea should join Russia.
"If Crimean residents vote for the accession of Crimea and the city of Sevastopol to Russia as a constituent entity, Crimea will be declared an independent republic after the referendum," the declaration says.
Seventy-eight of the 81 lawmakers present voted in favor of the declaration.
Meanwhile, Ukraine's parliament has warned the parliament in Crimea that it faces dissolution unless it cancels the referendum.
Western nations and the new government Kyiv have said they will not recognize the vote as legitimate.