The two European nations were unable to score until late in the game at the Estadio da Luz, when the Real Madrid forward broke the deadlock
The pre-match build up for the Estadio da Luz clash on Friday had been dominated by which Ballon d'Or hopeful - Ronaldo or Zlatan Ibrahimovic - would stand out for their side.
Both men failed to match their usual exceptional standards, but it was Ronaldo who earned the plaudits with a diving header in the closing stages to give his side a slender advantage ahead of the return fixture on Tuesday.
Portugal - who were looking for their first-ever victory over Sweden on home soil - had the majority of the possession in the first half, but it was the visitors who created the best chances, with Sebastian Larsson testing Rui Patricio and Kim Kallstrom going close with a 25-yard free kick.
Backed by a partisan home crowd, Portugal improved after the break and got their reward when Ronaldo stooped low to head home Miguel Veloso’s cross with eight minutes left.
Ronaldo returned to the starting XI after missing the 3-0 qualifying victory over Luxembourg through suspension, while Ibrahimovic was also back from a one-match ban for Sweden.
Portugal could have taken the lead as early as the fifth minute when Raul Meireles’ through-ball opened up the Sweden defence and Joao Moutinho rounded Andreas Isaksson, but from a tight angle he shot into the side-netting.
From the next attack, the hosts were almost caught out when Johan Elmander flashed an effort just wide from Mikael Lustig's centre.
It was Sweden who continued to create the better openings of the first half, and Patricio soon had to react well to a low drive from Larsson, before Kallstrom fired a free kick just wide of the post.
After struggling to create chances in the opening period, Portugal began the second half with more intent and after a bout of pinball in the Sweden box, Lustig had to be aware to clear on the goalline.
Nani - who had a quiet first half - sparked the game into life again in the 54th minute when he fired in a low cross that Per Nilsson put behind for a corner with Helder Postiga waiting to pounce.
The home side continued to create the better second-half openings and Postiga again flashed a near-post header from Moutinho's corner just wide.
They were finally rewarded in the 82nd minute when Miguel Veloso’s teasing cross was met by Ronaldo, who headed across Isaksson.
The game could have had more gloss when another Ronaldo header rebounded off the post with five minutes remaining but Portugal had to settle for their one-goal advantage ahead of the second leg in Solna.