Mercedes had not shown their full hand in the practice sessions, but were blisteringly fast once qualifying began, taking their third straight pole position of the season, and locking out the front row for the first time since the 2012 Chinese Grand Prix.
"I had a really good lap in the end...it's always a good motivation, especially for the team. Front row. Fantastic. But of course we have to be a bit cautious because of what we saw in Bahrain," said Rosberg, who finished ninth in that race.
"So it's nice, but to be enjoyed with caution."
Sebastian Vettel was third for Red Bull, ahead of the Lotus Kimi Raikkonen, with Ferrari's Fernando Alonso fifth in front of his home crowd.
Felipe Massa's final flying lap began well but faded to leave him sixth, while Romain Grosjean, Mark Webber, Sergio Perez and Paul di Resta rounded out the top 10 at the Circuit de Catalunya.
Q1 was a sedate affair, with teams eager to save tyres for later and run just once. In the quiet, Williams slipped silently out at the first hurdle.
For Pastor Maldonado particularly, given his pole and race win at the same venue last year, it represented a wretched slump in fortune.
"We've got to improve," said deputy principal Claire Williams. "You never panic in these situations. The grid is so tight, we were only one tenth off getting into Q2 so I don't think it's time to panic."
McLaren have suffered a similar fall from grace, with Jenson Button exposed in Q2. He eventually finished 14th, exposed for a lack of pace. But a surprisingly rapid lap from team-mate Perez saw him into the final session, with little to split the top drivers on the timesheets.
"For Jenson, it's a disappointing outcome," said principal Martin Whitmarsh. "We've not analysed the data yet but there is some evidence that he tyre pressures were too high otherwise we hope he would have snuck through."
There were four-tenths between the man in second and the man in tenth, but astonishingly Hamilton swept in with a late lap a clear six-tenths better than any of his rivals. The Mercedes could scarcely hide their smiles.
Knowing they had the edge on their rivals, they looked full of confidence in Q3. Rosberg went out early and set a lap time which only he himself went on to beat, while Hamilton plumped for one late run.
The true test of Mercedes will be their race pace - their drivers went backwards through the field from pole in China and Bahrain - but for now they appear untouchable in qualifying.
The three favourites for the title, meanwhile - Vettel, Raikkonen and Alonso - are all well-placed to push for a Grand Prix victory on Sunday.