With Real needing to beat Athletic Bilbao to win the championship, Ronaldo obliged with another match-winning display when it really counted.
The Brazilian, sporting the same bizarre hairstyle he wore at the World Cup, put his side ahead with a close-range strike after nine minutes and, after Bilbao had equalised, won the free kick that led to their second.
When news began to filter through that second-placed Real Sociedad were winning at home to Atletico Madrid and closing in, nerves were starting to fray at the Bernabeu but a brilliant second goal from Ronaldo settled matters.
Timing his run to perfection, Ronaldo latched on to a pass from Zinedine Zidane and belted the ball past Bilbao keeper Aranzubia to score his 23rd league goal of the season.
It was a first domestic league title for Ronaldo, who scored 34 league goals for Barcelona in the 1996-97 season but had to settle for the runners-up spot behind Real in the league.
"We're talking about the best player in the world," said Real coach Vicente del Bosque. "He has proved that and has helped us become the best team."
A tally of 23 league goals looked a distant target when he arrived from Inter Milan in a $44.2 million transfer.
His eight goals in South Korea and Japan, including two in the final, had made him the competition's top scorer and proved that he was back to something like his best after career-threatening injury problems.
Real's move for the Brazilian striker was still a huge gamble, though, with many questioning whether he could reproduce his World Cup form on a regular basis.
When he arrived at the Bernabeu, he was patently unfit and he did not make his debut until the fifth league match of the season against
With the eyes of the world on him, and many people waiting for him to fail, Ronaldo did what he does best, scoring two brilliant goals, one of them within a minute of coming on, to lead Real to a 5-2 win.
He has continued scoring vital goals for Real Madrid ever since, always saving his best for the biggest matches.
He returned to Yokohama, the scene of his World Cup triumph with Brazil, to win his first title with Real in December, scoring the opening goal in the side's 2-0 win over Olimpia of Paraguay in the World Club Cup.
Later in December he won the European Player of the Year award for the second time and the FIFA World Player of the Year award for the third time as he finished the year on a high note.
Ronaldo proved Real's saviour in the second phase of the Champions League, heading the only goal in a 1-0 win away to Lokomotiv Moscow that took his team through to the knock-out phase.
STANDING OVATION
In the quarter-finals, his hat-trick in the second leg away to Manchester United clinched a 6-5 aggregate win for Real and earned him a standing ovation from the Old Trafford crowd.
Ronaldo scored in Real's 2-1 win over Juventus in the semi-final first leg but injury limited him to a substitute's role in the return and Juventus denied the Spaniards a place in the final.
That left the domestic league as Real's only hope of claiming a major trophy and Ronaldo's inspirational run of form at the end of the season led them to victory.
He scored twice in a crucial 2-1 win away to Valencia, twice again away to Atletico Madrid last week and added another double to clinch the title against Bilbao.
"I'm so happy," Ronaldo told Real Madrid's website after the Bilbao win. "It's all worked out perfectly.
"When I was at Barcelona I was more spectacular but now I'm more effective. I'm getting better all the time."