Onyeka has penned a five-year deal with the club for an undisclosed fee after passing his medical.
Following the 23-year-old's arrival on Monday, Brentford head coach Thomas Frank said;
"He will now undergo a period of quarantine before joining with the squad for training.
“I am looking forward to getting Frank in the building. We think he is a strong addition to the squad, and we are looking forward to working with him. He is a very dynamic player.
“Frank plays as an eight in our system, he is very good at getting from box to box and will be valuable to us when we don’t have the ball.
"He has come from the Danish league and was one of the best midfield players in that league, we think he has the potential to develop further.
"All the coaches here are looking forward to working with Frank and pushing him to a higher level."
Brentford booked their ticket to feature in the top-flight for the first time following a 2-0 victory over Swansea City in the Championship Play-Off final in May.
Onyeka who has fearured just once for Nigeria is close to earning a call-up for the January 2022 Africa Cup Of Nations.
The Nigerian forward won two Superliga titles and one Danish Cup.
Onyeka made UEFA Champions League and UEFA Europa League appearances for the Danish side as leaves Denmark with123 appearances with 17 goals.
Rasmus Ankersen, Brentford FC Co-Director of Football, added:
“Frank Onyeka is a player we, for obvious reasons, know very well.
"Over the last two years he has grown to become the dominant midfielder in the Danish league and his performances in the Champions League last season convinced us that he is ready for the Premier League.
"With his pace, power and ability to win the ball I am sure Frank will be a great addition to the Brentford squad."
