Ukraine's veteran goalkeeper Andriy Pyatov kept Portugal at bay with several fine saves in the first hour before the hosts ran out of ideas in the Group B game.
"The team had some good moments and some difficult moments, we failed to take three or four chances in the first half," said Portugal coach Fernando Santos, whose side have a tough match at home to Serbia on Monday.
"Now we have to unwind and think about Serbia. It's a bit of a blow to our morale but the players will get over it."
Portugal had an early goal chalked off when William Carvalho's header was ruled offside and defender Pepe then produced an impressive volley which the 34-year-old Pyatov did well to turn over.
Ronaldo, left out of the Nations Cup matches in the autumn by mutual consent with Santos, then sprang into action as he twice got free on the left.
First he cut inside his marker and produced a snapshot which Pyatov saved with his legs and shortly afterwards he opted to go outside Serhiy Kryvtsov and fired in a low, left-foot shot which the goalkeeper again stopped.
The Ukraine captain saved his best for the 56th minute when he brilliantly turned away Andre Silva's goal-bound shot with a one-handed save.
With Bernardo Silva repeatedly dribbling into dead-ends, Portugal began to lose composure as both Joao Cancelo and Ronaldo sent wayward crosses straight into touch.
Portugal then had a let off when goalkeeper Rui Patricio blocked Yevhen Konoplyanka's shot -- Ukraine's only effort on target in the match -- and Ukraine debutant Junior Moraes saw his shot from the rebound blocked.
A late chance went begging for Portugal when substitute Rafa Silva missed his kick after a low Ronaldo cross found him in the area.
In the other Group B game, Luxemburg came from behind to claim a surprise 2-1 win over Lithuania.
Fiodor Cernych got behind the Luxemburg defence to open the scoring from a narrow angle in the 14th minute and Lithuania looked comfortable until Leandro Barreiro latched on to Vincent Thill's pass to level on the stroke of halftime.
Ten minutes after the break, Gerson Rodrigues netted with a curling, long-range shot to allow Luxemburg to spend at least a few days on top of the group.
(Writing by Brian Homewood; Editing by Toby Davis and Ken Ferris)
