The German's time of one minute 20.130 seconds was comfortably quicker than team-mate Lewis Hamilton's best lap of 1:20.558 on Saturday and the fastest in this year's Barcelona tests.
"I can feel that the balance of the car is good. I have the general feeling that we are in a better position than in the second half of last year but this is testing not racing, of course," said Rosberg.
Team principal Ross Brawn said Mercedes, who had a disappointing 2012 season when they finished fifth overall, had achieved their test objectives but also warned against drawing too many conclusions.
"I am pleased with the progress that the team has made over the last month and the steps forward that have been made with the car," he said in a team review of the session.
"We're realistic about our performance here and our position once we get to Melbourne, and we look forward to the challenge of a new season ahead."
Nine different drivers from five teams have led the timesheets in the 12 days of pre-season testing and all the signs are that the racing will be closer than ever, at least in the early races.
Champions Red Bull were fastest only once but are expected to remain the team to beat from the start on March 17 despite their low profile in testing.
Sebastian Vettel, who will be chasing a fourth successive world title, completed 100 laps on Sunday and set the eighth best time of 1:22.514.
"We're in good enough shape I think," he said. "Overall testing has been good for us and we didn't have too many problems.
"Over the past couple of days we maybe didn't tick all the boxes that we wanted to but we have bit of time left to prepare before the season starts. The target is there to improve and we will do that."
Ferrari's Fernando Alonso was second on the timing sheets at the Circuit de Catalunya, with 1:20.494, and McLaren's Jenson Button was third in 1:21.444 after completing an untroubled 122 laps.
"From the lap times we've seen, the season is going to be incredibly close," said McLaren sporting director Sam Michael.
Kimi Raikkonen returned to the Lotus cockpit after illness prevented him testing on Saturday, only for the car to turn sick on him with a gearbox failure.
"I was sick yesterday and the car was sick this morning. It's not what you want, but you go testing to find out if there are any problems with the car," he said.
"We haven't completed all the laps we wanted during pre-season, but that is often the case. We've found a few issues and we have solutions for them, which is positive...the feeling in the team is that we have a strong basis to start the season, even if we haven't shown that every day."
1. Nico Rosberg Mercedes 1:20.130s
2. Fernando Alonso Ferrari 1:20.494s + 0.364s
3. Jenson Button McLaren 1:21.444s + 1.314s
4. Nico Hulkenberg Sauber 1:21.541s + 1.411s
5. Kimi Raikkonen Lotus 1:21.658s + 1.528s
6. Paul di Resta Force India 1:21.664s + 1.534s
7. Pastor Maldonado Williams 1:22.415s + 2.285s
8. Sebastian Vettel Red Bull 1:22.514s + 2.384s
9. Valtteri Bottas Williams 1:22.524s + 2.394s
10. Charles Pic Caterham 1:23.115s + 2.985s
11. Jules Bianchi Marussia 1:23.167s + 3.037s
12. Daniel Ricciardo Toro Rosso 1:23.628s + 3.498s
13. Max Chilton Marussia 1:24.103s + 3.973s
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