Aaron Cresswell has extended his stay at West Ham by signing a new five-year contract.
Phil Jones has signed a new deal with Manchester United which keeps him at Old Trafford until June 2019.
The Russian-born Dutch 23-year-old joins the Scottish Premiership outfit after three seasons at Sparta Rotterdam.
United boss Jackie McNamara told the club’s official website that he was "extremely happy" about the appointment.
"He will bring a different dimension to our attack. He is strong, tall and quick and is at the right age to progress his career," he said. "Mario knows the players are developing here and wants to be part of that.
"We want the club to grow continually and with this signing we have enhanced our squad and improved our striking options."
Bilate, who is the club’s fifth signing of the summer after Jaroslaw Fojut, Callum Morris, Charlie Telfer and Blair Spittal, said: "This is a wonderful opportunity for me.
“United play attractive attacking football and the club will help me improve as a footballer.
"I am looking forward to showing what I can do. Scottish football will suit me and I want to play a big part in moving the club forward."
McNamara has money to spend after selling young playmaker Ryan Gauld to Sporting Lisbon for a reported £3million. Centre-half Gavin Gunning also left for Birmingham after his contract ran out in the summer.