The 25-year-old Senegal international has signed a four-and-a-half-year deal after completing a medical and agreeing personal terms, having arrived at the club's Finch Farm training ground earlier in the day.
"It's a great day for me, a big dream to play in the Premier League and a big dream to play for Everton," he told evertontv.
"Everton is one of the best teams in the Premier League and the Premier League is one of the best leagues in the world.
"I know Everton is one of the big clubs here in England with history, the fans and the great players, so I'm very excited and happy to be here.
"I started to watch Everton more since the beginning of October because I had heard about their interest.
"I was interested to see how they play and how they have good young players
If you see players who are aged 20 or 21 and see them perform like that, you know you have a good coach.
"The manager has said exactly what he wants
He believes I can be a player who can add something to the team."
Niasse, the Russian Premier League's player of the year in 2015, has scored 13 goals in 23 appearances this season and, as he can play across the front three, he comes in as a replacement for Steven Naismith, who was sold to Norwich for £8.5million earlier in the window.
He becomes the club's third most expensive signing after Romelu Lukaku (£28million) and Marouane Fellaini (£15million) and is expected to have the necessary visa clearance in order to be eligible for this weekend's Barclays Premier League match at Stoke.
Niasse will be presented to fans at the Toffees' home game against Newcastle on Wednesday.
"We're delighted because Oumar is a player we have been following for a long time," said manager Roberto Martinez.
"He is the right personality and someone who is in a very good moment of his career
He has had a lot of success in Russia and he brings a different quality to what we already have in our squad.
"I'm really pleased to be able to bring in Oumar in this window and we know he's going to have a massive impact going forward.
"He brings a real hunger and desire to be successful into what is already a strong squad."
Earlier in the day Everton loaned winger Aiden McGeady to Sky Bet Championship side Sheffield Wednesday but fellow Republic of Ireland international Darron Gibson opted to stay at Goodison Park despite Martinez previously suggesting the player would benefit from a loan to get some much-needed pitch time ahead of this summer's Euro 2016.
Source : PA
Source: PA