Kashima bagged a goal back with 12 minutes left but by that time Real were home and dry.
The win secures Real a place in Saturday's Club World Cup final.
Real have dominated European and World football in recent years - in part thanks to Ronaldo’s brilliance.
But the La Liga side sold Ronaldo to Juventus in the summer and Bale was effectively promoted to their lead man.
Gareth Bale celebrates his hat-trick for Real Madrid (Image: GETTY)
Gareth Bale is now Real Madrid's top star (Image: GETTY)
Bale followed Ronaldo - and Barcelona ace Luis Suarez - in beaching only the third player to ever score a hat-trick at the Club World Cup.
Real will not play Al-Ain in the final after the UAE side beat South American giants River Plate 5-4 on penalties yesterday.
Real have won the Club World Cup in three of the past four seasons.
The first claimed the trophy in 2014 when Bale scored in a 2-0 win over San Lorenzo.
Real beat Kashima in the final a year later - with the game this time going to extra time before a Ronaldo hat-trick secured a 4-2 win.
And last year it was Ronaldo who bagged the winner in a 1-0 result over Gremio.
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