The 22-year-old joined the Vikings in August 2015, and has scored 16 tries in 19 appearances so far.
"I am extremely pleased that Charly has decided to commit to us for the next two years," head coach Denis Betts told the club website.
"As a group he has settled in brilliantly. He is a fantastic bloke as well as a superb rugby league player."
Runciman played a big part in helping the Vikings retain their place in the Super League via last season's Super 8 Qualifiers.
And they have won six of their first seven matches this season to reach the top of the table.
"I have enjoyed my time here at Widnes Vikings since arriving last year and I am excited about what is to come in the future here," Runciman added.
"To be part of a club that proving that it is moving forward is really exciting for me and my team-mates."
Widnes Vikings centre Charly Runciman has signed a new two-year contract with the Super League club.