Manager Chris Hughton commented, "Jiri is a versatile forward, who is capable of scoring goals. He is a player who is able to play in any of the forward positions, and increases our attacking options for the second half of the season.
"He's a skillful and technical player, a full international, and has represented his country at every level. He has played in the top division in the Czech Republic and the Europa League."
"He is also someone we have been monitoring for some time, so we are delighted to have completed the transfer and to welcome Jiri to the club."
Born in Pardubice, Skalak began his career with Sparta Prague, and he signed his first professional contract with the club in 2010.
As a young pro he had a number of loan moves, including season-long spells with FC Zbrojovka Brno and Boleslav, before making the permanent switch to Boleslav last summer.
He has scored six goals in 16 games so far this season; overall he has scored 23 goals and made nine assists in 153 appearances.
Skalak will link up with his new Albion teammates for the first time tomorrow and will wear squad number 38.