The Barry Geraghty mount held off a stiff challenge from the Willie Mullins-trained Melon to come home a 6-to-4-on favourite to retain his title.
"He had a battle on his hands today and in a amusing way that's the first race he's had this year".
Altior will be in search of a 12th straight win, a streak that stretches back to 2015, but will likely do so on heavy ground for the first time in his career.
Jockey Barry Geraghty missed out on the Festival last season with an injury, but he was very impressed with the now six-time Grade One victor. "He won it the hard way, he battled hard. He definitely made two serious mistakes, if not three, and I thought he'd do well to finish in the first four, but he just kept picking up". "He proved there he's a better horse than past year". "He dug deep and scrapped it out".
Trusty partner Ruby Walsh, Cheltenham's most successful jockey, put in a tactical masterclass in the Arkle Trophy atop Footpad, timing the charge to perfection after Petit Mouchoir had set a rapid early pace to delight the fans in attendance. A jubilant Walsh, who returned to riding only a week ago after breaking his leg, punched the air and urged on the packed-out Cheltenham crowd after the win.
Mullins' Benie Des Dieux ran out a shock victor of the Mares Hurdle, as odds-on favourite Apple's Jade could only finish a disappointing third.
Summerville Boy (9-1) recovered from at least one shuddering error to claim a last-gasp victory in the curtain-raising Sky Bet Supreme Novices' Hurdle.
The concluding Close Brothers Novices' Handicap Chase went to Mick Channon's 13-2 shot Mister Whitaker, ridden by Brian Hughes. Willie Mullins' favourite Getabird finished unplaced.
The 5-to-1 favourite Coo Star Sivola claimed the Ultima Handicap Chase - the third race on the opening day of the Cheltenham Festival.