Puerto Rica's Trinidad pulled the fight out of the bag by comprehensively outboxing a tiring De La Hoya in the last three rounds. Judge Glen Hamada, of Washington, scored the fight 114-114, raising fears of another controversial tie in a high-profile contest. But his fellow American, Jerry Roth, saw it 115-113 and the Belgian Bob Logist gave it 115-114 to Trinidad.
Trinidad, who entered the fight as the IBF champion, took his record to 36 straight victories in this battle of unbeaten 26-year-olds, the richest fight ever outside the heavyweight division. De La Hoya suffered his first defeat after 31 wins.
Until those last three rounds, which Trinidad won convincingly, De La Hoya was edging a cagey fight with swift counter-attacks. Trinidad's best early round was the fourth, when he wobbled the American briefly from a big right and followed up with a short left. The WBC champion's left eye and nose were bloodied by the midway point, but his lightning hand speed and counter-punching moved him ahead on points.
Trinidad, though, forced the pace from round 10 onwards and De La Hoya went into survival mode as he offered little against the Puerto Rican's two-handed barrage.
A jubilant Trinidad said afterwards: "I always said from the very beginning that I was the No 1 welterweight and I was going to prove it. Oscar deserves a rematch."
De La Hoya insisted he thought he won the fight, but was magnanimous in defeat. The Los Angeles-based fighter said: "Obviously, in my heart I thought I won the fight. Next time, I will be a brawler. I wanted to demonstrate my boxing skills, but I guess it didn't work." He added that he would now take a vacation and think about his future.
On the undercard, Johnny Nelson exorcised the ghost of his last match in the United States by coasting to a successful third defence of his World Boxing Organisation cruiserweight title with a landslide points win over Tonga's Sione Asipeli.
Nelson overcame a poor start to outbox his brave opponent to earn unanimous verdict.
Nelson slunk out of the States on his last visit in 1992 after putting up an abysmal show in an IBF title challenge to James Warring. Though a vast improvement on that dark night in Virginia, it is unlikely that Nelson achieved his goal of winning over US fight fans.
The 32-year-old champion flattened Asipeli with a snappy right in the 10th round but the Tongan completed the mandatory eight count on his feet and continued to search out Nelson until the final bell.
The handful who bothered to turn up early for the appetiser on the bill will not have been particularly impressed with Nelson's typically careful and elusive style.
Indeed, the smaller Asipeli, 21, had the better of the early rounds, catching the champion with good right hooks as Nelson struggled to find his range. By the fourth, Nelson had learned enough about his opponent - who had won 15 of his previous 18 fights - to open up. But what little crowd there was still showed most interest when Nelson, who had slipped to the floor in round six, flipped back on to his feet.