Prieto spent five years as a youth player with Spanish giants Real Madrid before progressing his way into their B and C sides.BCFC
The 6ft 4in tall Prieto, a free agent, has agreed a three-year deal, with a year's option.
After the release of Lee Camp and the loan of Connal Trueman for the season to AFC Wimbledon, Head Coach Aitor Karanka has been looking to add to the senior goalkeeper ranks.
England youth international Zach Jeacock has been between the sticks for both Blues friendlies so far.
Born in the Spanish city of Alicante, Prieto spent five years as a youth player with Spanish giants Real Madrid before progressing his way into their B and C sides.
He then joined Espanyol B in 2014, where he would go on to make 80 appearances over the next three years, before moving to Malaga on two-year contract in 2017.
His first La Liga start for Los Albicelestes came against Barcelona in October 2017 in a 2-0 loss.
The 26-year-old departed Malaga in January 2019, having made four appearances and joined Leganes.
In 2019, he returned to one of his former clubs Espanyol on a two-year deal and went on to play three times for Periquitos, all in the Europa League.