Englishman Steve Webster is one stroke further back at minus-2 and was joined there by Sweden's Robert Karlsson.
Montgomerie, who leads the Order of Merit, got off to a hot start as he birdied the first and third. However, the Scotsman dropped a shot on the par-4 fifth. He bounced right back to birdie the sixth.
The 42-year-old holed out from the fairway for eagle on the par-4 eighth to jump to minus-4. Montgomerie, the 1993 winner, slipped to another bogey on the ninth, but recovered that stroke with a 10-foot birdie putt on the par-5 11th.
Montgomerie's tee shot to the par-3 12th was well offline. He missed the green well right and was unable to save par from there. The seven-time Order of Merit winner closed with a 7-foot birdie putt at the last to share the first-round lead.
'It's pleasing because the last few days have been difficult,' Montgomerie said. 'I've got everything to lose, but I love this golf course and I got rid of the anxiety in the first three holes. That's my second best score around here and I'm hitting it further, straighter and better than I've ever done.'
Poulter, who parred the first playoff hole last year to knock off Garcia, wrapped birdies on the fourth and sixth around a bogey on the fifth. He made the turn at 2 under as he birdied the par-4 ninth.
The Englishman parred the first four holes of the back nine at Valderrama. Poulter then birdied the par-4 14th. He got to 4 under with a birdie at 16 and managed to stay there as he got up and down for par at the last.
'The putter feels real comfortable in my hands right now and that is a good thing at this course,' said Poulter, a six-time winner on the European Tour. 'There was a slightly different wind from last year. It's pretty much opposite, which makes for a couple tricky tee shots on the par-3s. You just have think a bit more.'
Paul Casey and Niclas Fasth each carded rounds of 1-under-par 70. They share eighth place with Bradley Dredge, Gregory Havret, Raphael Jacquelin, Maarten Lafeber and Henrik Stenson. Lee Westwood and Nick Dougherty pace a group of eight players at even par.
U.S. Open champion Michael Campbell, who could pass Montgomerie to win the Order of Merit title, is five strokes behind the Scotsman as he opened with a 1-over-par 72, where he stands tied for 23rd.