The final quickly became a duel between the two Chinese divers, making sure that the world's pre-eminent diving power collected its sixth gold in seven events at the scenic Montjuic Municipal pool.
The difference between Si and Chen was non-existent until the fourth and penultimate dive that broke the deadlock in Si's favour.
Si became world champion in the youngster's first major international competition, while 2011 world champion Chen settled for silver to go with her gold in the 10-meter synchronized she won on Monday with Liu Huixia.
Si ended with 392.15 points, while Chen had 388.70. Ukraine's Iuliia Prokopchuk claimed the bronze with 358.40.
Si looked a lot more comfortable teetering in a handstand on the edge of a towering piece of concrete in front of thousands than speaking about her newly conquered world title.
"I fell pretty excited," she said, later adding that she considered herself an "introvert."
"I believe that my first four dives when I was pretty stable is what gave me the opportunity to win," Si said. "I feel pretty satisfied with all of my dives. I just had a little mistake in one of them. But overall I feel OK."
This was the 21-year-old Chen's eighth career medal at a world championship to go with her four Olympic golds. All have come in either the 10-meter platform individual or synchronized events.
Chen appeared to take her disappointing second place in stride and said she plans to continue on and compete in the 2016 Rio de Janeiro Summer Games.
"You can't be on top all the time," Chen said. "The key is how you face your failures. You have to always be looking toward to your long-term goals and not the immediate.
"After the 2008 (Olympic) cycle I almost wanted to give it up, but I worked hard. I think that this coming cycle will be easier for me. I welcome strong competition. It keeps me training hard."
Si joined her local diving team at the age of seven and has competed on the Grand Prix circuit since 2011. She had shown signs of being ready to compete for a medal after finishing the semifinals in third place.
Regardless, Chen was the favourite given both her outstanding medal record and her strong winning form here in Spain.
It was clear from the start that barring a major bobble from Chen or Si the title was going to come down to one of the two.
Chen and Si performed the same dive program and they both nailed their first three dives for identical scores each time to open up a gaping 42-point lead over the other 10 finalists.
After matching one another blow for blow, Si went ahead for good when Chen couldn't straighten out on a backward tuck with three-and-a-half somersaults on her fourth effort.
Si then executed a backward pike with two-and-a-half somersaults with one-and-a-half twists in her fifth dive to seal the win over her more experienced countrywoman.
Chen said that she erred on that critical fourth dive because she hadn't had enough time to get it down pat.
"I had only done that particular dive once before I came to Barcelona," she said. "I wasn't sure even up to the competition how to perfect that dive."
On Friday China looks poised to add yet another gold after 2007 champion Qin Kai and two-time reigning champion He Chong led Thursday's semifinal in the men's 3-meter springboard.
Qin and He already paired up to win the men's 3-meter synchronized springboard title on Tuesday, and the duo appears set to battle for the individual gold after Olympic champion Ilia Zakharov of Russia failed to advance to the final of 12.
Qin led the semifinal with 493.65 points through six dives, while He totalled 483.15.