It was the second in a row for Rosberg: the other was the third of his career and came ahead of the 2013 Bahrain GP.
ONE, TWO
Rosberg clocked 1min20.718, 0.254 better than team mate Hamilton. Three-times World champion Vettel was 0.336 behind Rosberg and will start on the second row alongside Kimi Raikkonen (Lotus).
Next up was Fernando Alonso and his team mate Felipe Massa (Ferrari, followed by Romain Grosjean (Lotus), Mark Webber (Red Bull), Sergio Perez (McLaren) and Paul di Resta (Force India).
Rosberg said later: "I had a really good lap in the end... it's always a good motivation, especially for the team. Front row. Fantastic. Of course we have to be a bit cautious because of what we saw in Bahrain; nice, but to be enjoyed with caution."
Mercedes last swept the front row at the 2012 Chinese GP, a race won by Rosberg while then team mate Michael Schumacher failed to finish.
Saturday was the first time since 1955, when Argentina's Juan Manuel Fangio and Britain's Stirling Moss were racing for the Silver Arrows, that a Mercedes works team had had three poles in a row. The automaker left F1 as a works team in 1955 and only returned in 2010 after buying title-winning Brawn GP.
Team principal Ross Brawn said: "Am I confident for tomorrow? Not entirely. I could see from fuel work that we're not on top of it as much as we are in qualifying but we've done a lot of work on that. Can we win? I don't know."
STRONG CHANCE
The statistics suggests Mercedes has a strong chance, given that only once in the last 22 years has a driver won in Barcelona without starting on the front row - and that was Schumacher in 1996 (from third).
Hamilton, denied pole in Spain with McLaren in 2012 due to a fuel irregularity, was fastest in the first two phases of qualifying but Rosberg made sure of the pole with a time of 1min20.718.
There was despondency at Hamilton's former team, McLaren, which is struggling to extract performance from its cars with 2009 World champion Jenson Button failing to make the final stage of qualifying for the first time in 2013. He will start 14th, with an upgrade package brought to this race failing to live up to expectations more than anyone could have feared.
The only glimmer of hope for McLaren was provided by Mexican Sergio Perez, who qualified ninth. Team principal Martin Whitmarsh said: "For Jenson, it's a disappointing outcome. We've not analysed the data but there is some evidence that tyre pressures were too high otherwise we hope he would have snuck through."
Further back, former champion Williams - 2012's race winners with Venezuelan Pastor Maldonado - were in even deeper trouble. A year on from the only win of his F1 career Maldonado qualified 18th with Finnish rookie team mate Valtteri Bottas 17th.
Team deputy principal Claire Williams said: "We've got to improve. You never panic in these situations. The grid is so tight, we were only a tenth off getting into Q2 so I don't think it's time to panic."
Qualifying from the F1 Spanish GP
1. Nico Rosberg (Germany) Mercedes 1min20.718
2 Lewis Hamilton (Britain) Mercedes 1min20.972
3 Sebastian Vettel (Germany) Red Bull 1min21.054
4 Kimi Raikkonen (Finland) Lotus 1min21.177
5 Fernando Alonso (Spain) Ferrari 1min21.218
6 Felipe Massa (Brazil) Ferrari 1min21.219
7 Romain Grosjean (France) Lotus 1min21.308
8 Mark Webber (Australia) Red Bull 1min21.570
9 Sergio Perez (Mexico) McLaren 1min22.069
10 Paul di Resta (Britain) Force India 1min22.233
Qualifying session 2 F1 Spanish GP at Circuit de Catalunya
1 Lewis Hamilton (Britain) Mercedes 1min21.001
2 Sebastian Vettel (Germany) Red Bull 1min21.602
3 Fernando Alonso (Spain) Ferrari 1min21.646
4 Kimi Raikkonen (Finland) Lotus 1min21.676
5 Mark Webber (Australia) Red Bull 1min21.718
6 Nico Rosberg (Germany) Mercedes 1min21.776
7 Sergio Perez (Mexico) McLaren 1min21.790
8 Felipe Massa (Brazil) Ferrari 1min21.978
9 Romain Grosjean (France) Lotus 1min21.998
10 Paul Di Resta (Britain) Force India 1min22.019
11 Daniel Ricciardo (Australia) Toro Rosso 1min22.127
12 JeanEric Vergne (France) Toro Rosso 1min22.166
13 Adrian Sutil (Germany) Force India 1min22.346
14 Jenson Button (Britain) McLaren 1min22.355
15 Nico Hulkenberg (Germany) Sauber 1min22.389
16 Esteban Gutierrez (Mexico) Sauber 1min22.793
Qualifying session 1 F1 Spanish GP
1 Lewis Hamilton (Britain) Mercedes 1min21.728
2 Nico Rosberg (Germany) Mercedes 1min21.913
3 Sebastian Vettel (Germany) Red Bull 1min22.158
4 Kimi Raikkonen (Finland) Lotus 1min22.210
5 Fernando Alonso (Spain) Ferrari 1min22.264
6 Mark Webber (Australia) Red Bull 1min22.342
7 Felipe Massa (Brazil) Ferrari 1min22.492
8 Romain Grosjean (France) Lotus 1min22.613
9 Paul Di Resta (Britain) Force India 1min22.663
10 JeanEric Vergne (France) Toro Rosso 1min22.775
11 Daniel Ricciardo (Australia) Toro Rosso 1min22.905
12 Adrian Sutil (Germany) Force India 1min22.952
13 Nico Hulkenberg (Germany) Sauber 1min23.058
14 Sergio Perez (Mexico) McLaren 1min23.116
15 Jenson Button (Britain) McLaren 1min23.166
16 Esteban Gutierrez (Mexico) Sauber 1min23.218
17 Valtteri Bottas (Finland) Williams 1min23.260
18 Pastor Maldonado (Venezuela) Williams 1min23.318
19 Giedo van der Garde (Netherlands) Caterham 1min24.661
20 Jules Bianchi (France) Marussia 1min24.713
21 Max Chilton (Britain) Marussia 1min24.996
22 Charles Pic (France) Caterham 1min25.070