The 35-year-old broke the deadlock in first-half stoppage time with his 31st league goal of the season and Federico Bernardeschi added the second as Juve moved seven points clear of Inter Milan.
Juve, in their first season under coach Maurizio Sarri, clinched the title with two games to spare despite a stuttering run in which they won only one out of five matches before Sunday.
Sarri, who claimed the first major Italian title of his career, blamed their problems on the unique situation caused by the three-month coronavirus stoppage which led to the championship race finishing in a hectic six weeks played at the height of summer.
Samp, who have climbed away from the relegation zone in the last few rounds and are 15th, had Morten Thorsby sent off for second bookable offence in the 76th minute.
Juve went ahead from a free kick when Miralem Pjanic rolled the ball to Ronaldo and the Portuguese swept the ball home.
Sampdoria had a flurry of chances early in the second half.
After those let-offs, Juve doubled their lead in 67th minute. Sampdoria goalkeeper Emil Audero could not hold Ronaldo's shot and Bernardeschi, who had replaced Danilo, snapped up the rebound.
Ronaldo's evening ended on a sour note when he smashed a penalty against the crossbar after Alex Sandro was fouled.