The 19-year-old, formerly on the books of Sporting Lisbon,, has signed a three-and-a-half year deal with the League One promotion chasers and follows on from the arrival of Yeovil winger Jordan Green, who joined the Reds on Friday.
The Reds have also signed a Uruguayan youth international striker on a deal until the end of the season.
On his signing, Portugal-born Simoes said: “I wasn’t expecting it to happen! I was surprised, I was heading to FC United training as normal, then I got a call telling me that Barnsley wanted to sign me.
“It is probably the best thing that has happened in my life. I knew I wanted to come straight away, I knew I wanted to join.
“The only worry you ever have is meeting new people and leaving home, but everyone has been great with me so far. I cannot wait to get playing now.”
Reds chief executive Gauthier Ganaye added: “We know we have secured a really exciting young player, on a long contract. This is really important for the future of the club and it also shows that we are attractive to young players, they want to come and develop here.
“Elliot is a player who will offer competition for first team places straight away. As already stated, he’s quick and exciting to watch on the ball.”
In addition, 20-year-old winger Mateo Aramburu will initially link up with the Reds’ Under-23s side, which is led by former Barnsley player Martin Devaney.
The right-footed frontman has played in three different countries – Uruguay, Argentina and France – and has represented his country at Under-15s, Under-17s and Under-18s level.
Aramburu was most recently with Le Touquet in the French third tier, where he has been working on his fitness securing a move to Barnsley.
Aramburu has signed a deal until the summer.
“I’m happy to have been given the opportunity to have signed here,” he said. “I’m looking forward to trying to achieve things with the club.
“I have already had the chance to meet the coaches and my new teammates who have been great with me, there are also excellent facilities here to help me develop. Now I want to work hard and impress to try and break into the first team.”