Mowbray said: "We've signed a young defender with a good pedigree whose talent has been nurtured for the last two years by Louis van Gaal [AZ Alkmaar's manager].
"Ryan's a player I've been monitoring for a few years and, alongside Gianni, he was part of the Dutch Under-21 team which won the European Championships last year.
"Although Ryan's still only 22, he has a huge physical presence and I'm delighted to have added him to our defensive options.
"Leon Barnett and Abdoulaye Meite have done very well in central defence but everyone saw on Saturday how stretched we were in that department when Abou went off injured and Gianni had to move into the middle.
"We've now got plenty of competition for places in that area and I'm sure the fact Ryan knows Gianni really well will help him settle in quickly."