The 25-year-old, known for his pace and power, penned a four-and-a-half year contract, much to the delight of manager Roberto Martinez.
"He is the right personality and someone who is in a very good moment of his career," Martinez told the club's website (www.evertonfc.com).
"He has had a lot of success in Russia and brings a different quality to what we already have in our squad."
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Niasse, named Russian Premier League Player of the Year in 2015, joined Lokomotiv in 2014 from Turkish club Akhisar Belediyespor, scoring 19 goals in 42 appearances.
He cannot make his English Premier League debut against Newcastle United on Wednesday but could be in line for Saturday's trip to Stoke City once international clearance has been obtained.
Niasse has won four caps for Senegal.
(Reporting by Martyn Herman; Editing by Tony Jimenez)
