Forest boss Steve Cooper wants to strengthen his defensive options and has turned to Aina, who has departed Italian side Torino, as an addition.
The Premier League club are rushing to push the deal over the line and Aina is flying into Spain today.
Nottingham Forest will then put him through his medical paces this evening and if he comes through without an issue, he will join the club.
Aina knows English football and came through the youth ranks at Chelsea, before departing the club permanently for Torino in 2019.
The defender spent the 2020/21 campaign on loan back in England at Fulham.
Now he is set to return to England and link up with Nottingham Forest ahead of another season of Premier League football at the City Ground.
Aina, who turned out for England at multiple youth levels, is a senior international for Nigeria.