The world record holder and Olympic champion ran 46.89, pulling away from American Rai Benjamin heading into the home straight.
Warholm won the 2017 and 2019 world championships but struggled to seventh at the worlds last season in Eugene when he was hampered by an injury.
"It feels incredibly good to have the gold around my neck again," Warholm told Norwegian broadcaster NRK. "I fought all I could in the last 100 metres."
Kyron McMaster won silver in 47.34 to earn the first world championships medal for the British Virgin Islands.
"This means the world to my country - I have been chasing this medal since 2017," McMaster said.
"For so many years, I have been chasing a medal, but (it is) not the ultimate medal, because that is a gold."
Benjamin, silver medallist both last year in Eugene and at the Tokyo Olympics, faded to take bronze in 47.56.
"I do not know what happened. I am proud of myself but I just wanted more. I want to prove I have it in me," Benjamin said.
"It has been a very tough season, dealing with a quadriceps injury, a lot of changes, a lot of distractions. I lost a close friend of mine. It was just tough."
Warholm, running out of Lane 7, and Benjamin, on his inside in Lane 6, hit through the first seven hurdles in perfect synchronization before the Norwegian found another gear.
"I know that there is no one else who can do the first part of the race as fast as I can. And if they do, they will have a hard time at the end," said Warholm, who was given a Viking hat from a Norwegian fan during his victory lap.
"Today, unlike other races I've had, I felt like I was a bit sneaky and was waiting. And then I took it at the end, which people don't think I can do. But I also have it in the toolbox when needed."
Warholm's time of 45.94 in Tokyo obliterated his own world record, but since tearing his hamstring six weeks before last year's worlds, he has had to make a steady climb back to the top.
He has been unbeaten this season and hurdled to a season-best 46.51 at the Monaco Diamond League on July 21st, the second-fastest time of his career.
Moon and Kennedy share pole vault title
American Katie Moon (32) and Australia's Nina Kennedy (26) decided to share the gold medal in a dramatic women's pole vault final at the World Athletics Championships on Wednesday.
The two women cleared 4.90 in an event that stretched two hours and 10 minutes, but both missed on all three attempts at 4.95 and decided to share the victory rather than go to a jump-off.
It was the second consecutive world title for Moon, who also won Olympic gold in Tokyo. Armed with a personal best of 4.95 and a season's best of 4.90, Moon appeared to be the favourite.
But Kennedy had other ideas. She surpassed the Australian record and her own personal best by eight centimetres when she sailed over 4.90 and then held back tears as she looked up at the steady bar in joyful disbelief.
The two chatted briefly to decide the outcome, then embraced, conjuring memories of the Tokyo Olympics where good friends and rivals Mutaz Barshim of Qatar and Gianmarco Tamberi of Italy opted to share men's high jump gold.
"I am a big softie, it is so beautiful, it really displays what our sport is about," Britain's 2012 Olympic heptathlon champion Jessica Ennis-Hill said on the BBC.
"Two epic performances and they've both come away with that gold medal."
Wilma Murto (25) of Finland tied her season's best of 4.80 for bronze, missing all three of her attempts at 4.85. She became the first Finnish woman to win a Diamond League title when she beat Moon at the London event on July 23rd.
The event captivated the National Athletics Centre crowd who - prompted by an ominous rhythmic thumping over the sound system when each jumper stepped up to the runway - clapped in time.
