The Seagulls have paid Preston an undisclosed fee for 20-year-old winger Monakana, who will initially play for the club's development squad.
The former Arsenal trainee told Brighton's website: "Rohan said he took a little time to adapt - but now he has and having been given a chance to play, he has taken it.
"He advised me to do my best in training and in games and that he is sure I can break through as well.
"In the summer, we often do fitness programmes together and I played against him a few times when he was at Chelsea and I was at Arsenal.
"It is not really a surprise to see he is doing what he is doing now. My aim is to go on and do the same as him."