The 20-year-old forward was a free agent following the end of his contract in Spain and has put pen to paper on a three-year deal.
Mujica, who scored seven goals in 33 games for Barca’s reserve side last season, will initially feature in the Whites’ Under-23 side who won the Professional Development League title last term.
Meanwhile, Leeds have also added Barca schemer Guillermo Amor on a free transfer.
The midfielder, 18, has agreed a two-year deal and will initially join up with the club’s Under-18’s.
Hull City and Barnsley are battling it out to sign winger Mallik Wilks, who is set to leave the Elland Road club shortly.
Doncaster Rovers will conduct interviews for the vacant managerial position on Monday and Tuesday – with a view to an appointment later next week.
A number of names have been mooted as potential candidates including former captain Darren Moore, ex-Oxford United chief Michael Appleton – who lost out to Paul Dickov when Rovers appointed a new manager in May 2013 – Motherwell manager Stephen Robinson and former Barnsley and Mansfield boss David Flitcroft.
Rovers chief executive Gavin Baldwin said: “We have had strong interest and have taken references across all levels.
“We did not choose for Grant (McCann) to leave, but it is our job to ensure we come out the process in a better position than we went into it and we are extremely optimistic.”
Ex-Bradford City defender Nathaniel Knight-Percival has joined Carlisle United on a one-year deal.
Middlesbrough have re-signed goalkeeper Tomas Mejias – who played for the club between 2014 and 2018 – on a free transfer with the Spaniard signing a two-year contract.