Kane, a second-half substitute, beat Juventus goalkeeper Wojciech Szczesny with a first-time strike with his right foot after latching onto a pass from Lucas Moura, who had earlier made it 2-2 with a goal to cancel out Ronaldo's close range effort which had made it 2-1 to the Italian champions.
"It's probably one of the best goals in my career," Kane said. "I saw the keeper was quite far off his a few times, so I thought I would take the chance if I had the opportunity. I saw him off his line and fortunately it went in."
Spurs had taken the lead in sweltering temperatures in Singapore, with both teams stopping for three minute drinks breaks in each half, following Erik Lamela's opener.
Gonzalo Higuain levelled for Juventus in the second-half before Ronaldo brought the crowd to its feet with his goal before being replaced after an hour of his first preseason outing.
I think it was a great goal. Unbelievable from Harry," Spurs boss Maurio Pochettino said. "I think it is good for him to start the preseason scoring. I think to beat Juventus of course that wasn't the priority to win the game but to compete in our best way and in the end to win is always better than not winning."
Juventus, having only arrived in South East Asia on Saturday afternoon, were surprisingly competitive against a Spurs side that have been back in preseason for a week longer than Maurizio Sarri's team.
The game also saw £67.5 million new arrival Matthijs de Ligt make his Juventus debut following his move from Ajax, but Kane's late strike ensured that the Dutchman suffered stoppage time defeat against Spurs for the second time in three months having also been on the wrong end of a 3-2 loss in the Champions League semifinal second-leg against Mauricio Pochettino's team for Ajax.