The 24-year-old prop has signed an 18-month deal, visa permitting, and should stay at the club until the end of the 2017 season.
Since he made his Tigers debut in 2013, he has played 41 NRL games.
“This is a great opportunity to come up for me half way through the year and I am really looking forward to coming over,” Buchanan told the Vikings’ official website.
“I have been watching the team when I can, they don’t air all games over here but they look like an exciting bunch of players that I can’t wait to get over and work with.
“Charly Runciman is a big part of why I am coming to the Vikings. I played with him for two years when I was younger and speaking to him has made me really excited to get over.”
Widnes head coach Denis Betts believes Buchanan will add stamina and ambition to the Vikings pack.
“Jack is a young big front rower with a great engine,” he said.
“He has significant NRL experience and is ready to make his mark, looking to establish himself as a starting front rower and wants to do that here at Widnes.
“Again this is a big statement from us as a club. Jack is an enthusiastic player that wants to come here and progress his career making himself into a better player.
“He is a great signing for us.”