Benfica, beaten on penalties by Sevilla in Wednesday's Europa League final, had already won this season's league and the League Cup in which they also beat Rio Ave in the final.
Benfica dominated the first half at the Jamor, the old National Stadium which continues to regularly host the Cup final, and Gaitan put them ahead with a shot from just outside the area in the 20th minute.
Rio Ave, also beaten on their only other appearance in the final 30 years ago, could not muster a shot before halftime but the second half was a different story.
Oblak twice denied Ukra, once with his feet and then with an acrobatic stop, while Pedro Santos struck the post. The 21-year-old goalkeeper again came to the rescue in stoppage-time with a brave save at the feet of Ahmed Hassan.
"We were clearly superior in the second half and deserved a better result," Rio Ave coach Nuno Espirito Santo told RTP television.
Benfica coach Jorge Jesus said the Europa final which went to extra-time, had taken its toll on his team.
"We wouldn't have had the energy to chase the goal if we hadn't scored in the first half," he said. "The players were worn out."
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