A total of 120 passengers were onboard Douro Elegance across two eight-day Douro, Porto and Salamanca sailings.
The cruises featured excursions to a Porto vineyard and the old city of Salamanca, as well as guided tours of local landmarks including Castelo Rodrigo and the Mateus Palace gardens.
Riviera will now operate a series of river cruises across Europe up to early November.
Guest Sally Harper described Rivera’s organisation as "absolutely second to none".
"I can’t fault it," she added. "This is the second trip we’ve done with Riviera, the first one was Croatia, but this is the first river cruise I’ve ever done and it’s gone beyond my expectations.
"Everybody told us to expect a beautiful river and this has just blown me away. The food is great and the staff are so friendly, everywhere is so immaculately clean. We’ve felt really safe and secure at all times."
Katja Hildebrandt, head of product river and ocean cruise at Riviera Travel, said the Douro cruises were the "perfect first sailings back".
"Our guests really did feel they were away-from-it-all as they visited some of Iberia’s most beautiful towns, gardens and vineyards," she added. "We can’t wait to welcome more people back onboard, as our European river cruises return to some of the most beautiful destinations along the likes of the Seine, Rhone and Danube."
