Nottingham Forest have signed Murillo from Brazilian club Corinthians, with boss Steve Cooper saying a deal to sign Nuno Tavares is "at the final stage".
Defender Murillo, 21, has signed a five-year deal, after the clubs agreed a fee of around £15m.
Meanwhile, Portuguese left-back Tavares, 23, looks set to join Forest on a season-long loan from Arsenal.
But Cooper says there is "no official news" on reports linking forward Brennan Johnson with a move away.
Murillo, who broke into the Corinthians first team in April and has played 13 times in Brazil's top flight, said he was "excited" to join Forest as their sixth signing of the summer.
"The Premier League is the best league in the world, so I can't wait to test myself and give my all for this club," he added.
Wales international Johnson has repeatedly been the subject of transfer speculation, with Tottenham and Brentford both thought to be keen to sign the 22-year-old prior to Friday's summer transfer deadline.
However, speaking before the Murillo deal was officially announced, Cooper tried to play down the prospect of Johnson's imminent departure.
He said: "Maybe there's some things that are being speculated that are very true and some that couldn't be any further away from the truth.
"I do believe there's a couple of players, Tavares and Murillo, that are at the final stages. Not quite done, but the process is ticking along quite nicely.
"I hope I haven't gone too soon on that because until things are officially done then you never quite know. We shall see. We just have to carry on as normal."
Tavares has made 28 appearances for Arsenal in all competitions since joining them from Benfica in 2021 and spent last term on loan at Ligue 1 side Marseille.
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