The Bristol-born 31-year-old joined Harlequins from his hometown club in 2006 and has gone on to make over 100 appearances for the English Premiership Champions.
Kohn was an integral part of the Quins side that lifted the title in 2012 and is eligible to play for Wales due to his Welsh grandfather.
He will join up with the Welsh squad on Sunday evening and Howley is looking forward to welcoming him into the camp.
"We looked at Olly when we initially selected the Six Nations squad and we have brought him in now as we believe he will be an experienced addition," said Howley. "He has been performing very well for Quins and has been a feature in their pack.
"He's a strong ball carrier and dominant at the set piece and it will be good to see him in the international environment."
Kohn was delighted with the announcement and added: "Receiving the call up was surreal and I can't wait to join up with the squad this weekend.
"It's a really proud moment for me and a great opportunity to show what I can do. It will be good to meet the squad, get training and hopefully be part of a successful Six Nations campaign with Wales."
Height: 2.01m / 6' 7"
Weight: 138kg / 21st 10lb