Fernandes will join Quique Setien's squad from Palmeiras in July after a €10 million ($16.3 million) package was agreed for the 21-year-old, who has played for Brazil Under-20.
Braga accepted a €31 million ($50.6 million) offer from Barca for 20-year-old forward Trincao, and he will also formally join the Catalan club's ranks at the end of the season.
Both players have agreed five-year contracts, the Blaugrana announced, with Fernandes's contract including a €300 million($489 million) buyout clause and Trincao's deal showing a €500 million ($815 million) clause.
Braga will take Barca's 18-year-old striker Abel Ruiz on loan until the end of the season and then make the deal permanent for €8 million ($13.1 million), with the LaLiga club having a buy-back option.
Barca was not the only Spanish club busy in the market though, with Celta Vigo signing striker Fyodor Smolov on loan from Lokomotiv Moscow until the end of the season.
Atletico Madrid re-signed Belgian winger Yannick Carrasco, almost two years after he left to join Chinese side Dalian Yifang. The 26-year-old joined until the end of the season on loan.
Deadline day was as notable for the deals that looked to have fallen through as those that were completed.
Paris Saint-Germain ruled out allowing Edinson Cavani to leave, as a host of clubs fancied taking the Uruguayan off its hands, while Chelsea refused to let France striker Olivier Giroud depart Stamford Bridge.
Tottenham Hotspur was among the clubs keen on Giroud, but Jose Mourinho looked like missing out on the striker he wanted to strengthen a frontline depleted by the loss of injured England captain Harry Kane.
Real Madrid boss Zinedine Zidane said Gareth Bale would not be leaving his squad.
Manchester United remained linked with Bournemouth forward Joshua King, but time was running out for Ole Gunnar Solskjaer to add new faces to his Old Trafford squad.
United made one signing, but it was a low-profile deal as England Under-20 goalkeeper Nathan Bishop arrived from third-tier Southend United.
Arsenal brought in defender Cedric Soares on loan from Southampton.
In Scotland, champion Celtic shipped out Scotland international Lewis Morgan to Major League Soccer side Inter Miami, which is part-owned by David Beckham.
Belgian side Genk allowed attacking midfielder Ianis Hagi, son of Romanian great Gheorghe Hagi, to join Scottish giant Rangers on loan.
KOUAME, DUNCAN IN
Serie A transfer activity was dominated by Fiorentina, which signed Ivorian forward Christian Kouame from Genoa, initially on loan. The deal will be made permanent at the end of the season.
It also brought in Ghanaian midfielder Alfred Duncan from Sassuolo in a similar deal, a loan with an obligation to buy.
Reports in Italy said Kevin-Prince Boateng was set to leave Fiorentina among the incomings, with Besiktas his destination. Turkish giant Besiktas also snapped up 18-year-old Croatian midfielder Ajdin Hasic from Dinamo Zagreb on permanent terms.
Atalanta agreed a loan deal with Nice, acquiring defender Adrien Tameze from the Ligue 1 outfit with a purchase option.
FIFA World Cup winner Steven Nzonzi cut short a season-long loan from Roma to Galatasaray, returning to his French homeland with Rennes on a six-month loan with an option for a further year.
In the Bundesliga, Emre Can left Juventus for Borussia Dortmund on Saturday (AEDT, initially on loan until the end of the campaign but with a €25 million ($40.8 million) fee agreed for a potential permanent transfer.
Elsewhere, Hertha Berlin followed up Friday's (AEDT) signing of Krzysztof Piatek by prising 20-year-old Brazilian winger Matheus Cunha from table-toppers RB Leipzig, for which he scored nine goals in 52 games.
Hertha managing director Michael Preetz said: "We wanted to increase the quantity and improve the quality up front, which we have been able to do with the Piatek and Cunha transfers."
Leipzig also offloaded Austrian midfielder Stefan Ilsanker to Eintracht Frankfurt. He spent four-and-a-half years at Leipzig, joining it in the second tier.
Werder Bremen signed striker Davie Selke on an 18-month loan from Hertha, the 25-year-old returning to the club where he began his professional career.