The 25-year-old joins the club from French league champions Rapaces de Gap where he played alongside two other Lightning signings – Kyle Essery and Kevin King.
After being drafted by Buffalo Sabres in 2010 the American spent four seasons playing in the NCAA with the University of Minnesota, helping them to win two regular season championships.
He went on to ice for Colorado Eagles and Wichita Thunder in the ECHL before moving to France last season.
Lightning boss Pete Russell said: “I’m delighted to have secured Christian as there was a lot of interest in him around mainland Europe.
“Christian is strong, is a good skater, really good hands, can score and plays both ways. He uses his body well and is strong along the walls, competes hard and has a good work ethic. I really like his overall game.
“During the second half of last season in France, he really came into his own. He really found another level in his game and was a big, big player for Gap.
“During the play-off run he was among the top five overall point scorers in the country.
“Christian can play in all situations and can really make a difference. He is another very exciting addition and adds another dimension to our line up – I’m absolutely delighted to welcome him to the Lightning.”
Isackson added: “This is going to be a great year – I can’t wait to be a part of this club and making some noise as we begin our first year in the top flight in the UK.
“I’m very excited to be joining and look forward to meeting everyone next month.”