Ronaldo got on the scoresheet at last as the Portuguese put themselves in prime position to accompany Brazil into the second round after a scintillating display of attacking football.
Portugal coach Carlos Queiroz had said he would approach the game in three stages - win, score goals and then look at the standings - and his players followed the plan to the letter.
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Goals by Raul Meireles in the first half, Simao Sabrosa, Hugo Almeida and Tiago in a seven-minute blitz early in the second, and further strikes by substitute Liedson, Ronaldo and Tiago again secured the win that took them on to four points.
"It was a great display, good goals, great attitude. We needed a moment like this, a well-played game so that the motivation and confidence can be even bigger now," said Queiroz.
"It's a big result, but it's another game and we still haven't won anything yet. Now we have to think of qualifying for the next round, one game at a time, try to win the next one with the same ambition with which we approached this game."
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Portugal are now two points behind already-qualified Brazil, who they meet in their final game, but more importantly have boosted their goal difference so much that Ivory Coast need to score a hatful in their final game against the Koreans.
The Ivorians, beaten 3-1 by Brazil on Sunday, have one point and a draw for Portugal in their final game would in any case guarantee that Queiroz's side progress to the last 16.
North Korea's amazing journey, though, has suffered a terminal blow and coach Kim Jong-hun said he took responsibility for the heavy defeat for "not planning the right strategy.
"...our players, I think, played to their full potential," he said. "However, tactically speaking, I think it fell apart so we couldn't block their attacks and that's why they scored a lot of goals.
RELENTLESS RAIN
North Korea had earned widespread praise for their battling defensive effort against Brazil but, contrary to most predictions, they were far more positive on Monday.
Although Portugal should have gone ahead after seven minutes when goalkeeper Ri Myong-guk failed hopelessly to claim a Simao corner and Ricardo Carvalho thumped his header against a post, it was anything but one-way traffic.
Conditions were made treacherous by relentless rain and with defenders slipping and the ball skidding, both sides unleashed shots from long range at every opportunity.
Cha Jong-hyok fizzed one just wide while a parried Hong Yong-jo effort fell to Pak Nam-Chol but he headed over.
After 25 minutes of their famous 1966 quarter-final North Korea led 3-0 before eventually losing 5-3.
