Williams polled 7,796 votes on the BBC Sport website, three times more than his nearest rival, Ireland captain Brian O'Driscoll with 2,466. Tricky Wales winger Shane Williams came third with 2,066 votes.
The 29-year-old, who might not have started a game in Wales' Grand Slam campaign had Colin Charvis been fit, said: 'It's a great honour to be named RBS player of the championship especially as this is voted for by rugby fans.
'This award caps a tremendous RBS 6 Nations for Wales and is as much a vote for my team-mates as for me.'
He added: "It has always been an ambition of mine to be part of a Grand Slam side.
"It has also been a dream of mine to be able to have the last kick of a championship-winning game, just like I did on Saturday when I kicked the ball into touch to signal the end of the game."