Having not featured for Portugal since its defeat to Uruguay at the FIFA World Cup in 2018, Ronaldo was back fr this encounter in Lisbon, but failed to inspire a victory as the defending European champions began its Group B campaign on a flat note.
Andriy Pyatov played a key part in frustrating the hosts, twice denying Ronaldo in the first half before pulling off an exceptional save from Andre Silva.
Pyatov's heroics paid dividends as Ukraine held firm to secure a hard-fought point, with the onus now on Portugal to respond when it faces Group B's other big hitter, Serbia, on Tuesday (AEDT).
Pyatov did well to tip over Pepe's blistering effort, though Ukraine's goalkeeper was fortunate that the offside flag sparred his blushes moments later after his failure to deal with a cross allowed William Carvalho to head in.
Pyatov atoned soon after, twice reacting sharply to keep out successive close-range strikes from Ronaldo and maintain parity heading into the interval.
Ronaldo turned provider with a neat knockdown after the restart, but Andre Silva's resulting effort did not test Pyatov.
The Sevilla forward did better when his next chance came, only for Pyatov to make a superb save at full stretch.
Portugal was almost made to pay for its profligacy with four minutes remaining, but Ruben Dias made a vital block to prevent Junior Moraes clinching the points for Ukraine against the run of play.
Vitaliy Mykolenko risked undoing Ukraine's work in stoppage-time when he clumsily bundled into Dyego Sousa, though Portugal's appeals for a penalty fell on deaf ears.