The 2012 Olympic champion, taken to 12 roundsfor the first time in his professional career after 20 previous knockouts, added Parker’s WBO belt to his own WBA Super, IBF and IBO titles.
In their title unification bout, the three judges scored it 118-110, 118-110 and 119-109 to Joshua, leaving only Deontay Wilder’s WBC title still to conquer.
“Wilder. Let’s go baby, let’s go,” shouted Joshua from inside the ring to raucous cheers from the 78,000 crowd packing Cardiff’s Principality Stadium, also name-checking Tyson Fury as another possible opponent.
“Do I want to become undisputed heavyweight champion of the world? IBO, WBO, IBF, WBA…I’m not into the hype, I’m about business. So let’s get the business done.
“Let’s see what the future holds, I’m down for whatever, whenever,” added the Briton, making clear he wanted Wilder to face him in Britain rather than Las Vegas.
The American was not among those present, in spite of being invited by organisers, the undefeated champion opting to watch from afar instead.
Both fighters had stepped into the ring undefeated, with Joshua weighing in six pounds heavier than his shorter opponent and with a longer reach.
If some had expected an early knockout it did not pan out that way, even if Joshua could be said to have controlled most of the 12 rounds.
This he did by using his jab and left hooks to telling effect.
Apart from an explosive sixth round, with both trading some stinging blows, neither looked likely to buckle with Parker showing more staying power than any of Joshua’s previous opponents.
The referee also intervened repeatedly, breaking up the fight whenever the fighting livened up.
He called a time out in the ninth with Joshua returning to his corner to tighten up his gloves.
“My strategy in there was to stick behind the jab,” said Joshua.
“It’s one of the most important weapons. The old saying is the right hand can take you around the block, but a good jab will take you around the world. And that secured me another world title belt,” he added.
“I know the bookies say I might get a knockout…but forget the hype, Joseph Parker is a world champion. So I knew he was going to be determined. So this does become a boxing match, not a fight.
“Joseph Parker stated this would be war, I stated this would be boxing finesse. I stuck to my word and I controlled it behind the jab, the counter-punches.”
The 26-year-old New Zealander had been led into the ring by Samoan chiefs under a barrage of strobe lights.
This was before Joshua walked in to the sound of ‘Paid in Full’ and ‘Another One Bites the Dust’.
He was however gracious in defeat.
“Today I got beaten by a better champion, a bigger man,” he said.
“I’m going to come back stronger. No regrets, take it on the chin. Better man on the day.
“I would love to have another go.”