The opening exchanges were frantic as both sides looked to establish dominance, but it was Spain who struck first after 10 minutes as they fought back from their defeat to Egypt last time out.
Barcelona's Lopez picked up a loose ball 25 yards from goal and swivelled before unleashing a rocket that got the better of Leo Kokubo despite the goalkeeper getting a firm hand on the shot.
Despite having the lead, Spain appeared to take their foot off the gas and were almost made to pay shortly before half-time.
Japan's Mao Hosoya had plenty to do as he received the ball with his back to goal, but he was able to turn and his snap-shot was drilled past the despairing dive of Arnau Tenas, only for a VAR review to show his standing foot was fractionally offside when the ball was played to him.
Hosoya’s luck appeared to be out as he came close again just before the interval but his back-post header hit the inside of the post and bounced to safety.
That escape combined with the break allowed Spain to reset and Alex Baena had a great chance to make it 2-0 as he found space in the box, but he dragged his effort well wide.
Although Japan were pushing hard, they couldn’t fashion any clear-cut chances and they were stung by another Lopez stunner with 15 minutes to go.
Sergio Gomez picked him out on the edge of the box from a corner and the Barcelona forward chested the ball down before unleashing a stunning volley past Kokubo.
A third goal was harsh on Japan but it came in the final minutes when Abel Ruiz pounced on a loose ball in the box before crashing it into the back of the net to make it 3-0.
It was a disappointing defeat for Japan as they were perhaps the better side for parts of the game and saw Kota Takai’s late header cannon off the bar.
However, their search for an Olympic medal goes on as Spain now face Morocco with a chance to add yet another international honour to their European Championship success earlier in the summer.
Flashscore Man of the Match: Fermin Lopez (Spain)
