Both men had to overcome cuts around their left eye in the opening stages in Saudi Arabia.
But it was Joshua who produced the more disciplined performance in the desert as he established a comfortable lead by the halfway point.
Ruiz began to work his way into the fight in the second half but was unable to get close enough to land the telling blows.
And the Brit eased through the remainder before being given the nod by all three judges as he once again sits on top of the world.
Official result: Anthony Joshua def. Andy Ruiz by unanimous decision (118-110, 118-110, 119-109)
He’s once again the WBA, IBF, and WBO heavyweight champion of the world.