Garcia, who won the last staging of the event in 2011, carded seven birdies and two bogeys in an opening 66 to finish five under par alongside Holland’s Joost Luiten.
Fellow Spaniard Pablo Larrazabal and England’s Robert Rock were a shot off the lead, but world number five Jon Rahm struggled to a three-over-par 74 in his first professional tournament on home soil.
Masters champion Garcia started on the back nine and birdied the 10th, 12th, 15th and 18th to reach the turn in 32, before following a bogey on the first with three birdies in four holes from the third.
At that point the 37-year-old enjoyed a two-shot lead, but a wayward drive on the eighth led to a second bogey of the day as Luiten came home in 31 to join Garcia at the head of affairs.
“It was unfortunately very wet after the rain yesterday and this morning but the course is holding up well and towards the end of the round the ball wasn’t even picking up mud on the fairways,” Garcia said.
“Obviously it was gettable because it was soft but you still have to hit good shots and I was able to do that for the most part of the day. I hit my irons very well and drove the ball well other than on the eighth hole. I made some nice putts here and there and overall it was very solid. I’m very happy with my round.”
Luiten, who was second behind Andrew Johnston when Valderrama hosted the Spanish Open last year, also started on the back nine and was only level par after 11 holes, but birdied five of the next six.
“Early on I thought it was playing tough and you have to be patient,” Luiten said. “Finally on my back nine I started to hit some really good iron shots and hit some close for easy birdies.I hit four shots within three feet and that makes this game a lot easier.”
Johnston was three under par after eight holes but eventually had to settle for an opening 69, while former Masters champion Danny Willett was a shot behind on one under.