The England midfielder, who has been struggling to recover from an injury to his left ankle, was cheered loudly by the capacity crowd of 27,000 at the Home Depot Center as he ran on to the pitch in place of striker Alan Gordon in the 78th minute.
"It's incredible, that is what we want to achieve," the former England captain told ESPN television. "The atmosphere, the full stadium, it's great.
"It is a great day for the club and it is great to have a club like Chelsea out here."
His problem ankle was another matter, though.
"I was pleased to get out there but I'm not fit and I haven't trained since I have been out here," Beckham said.
"It (the ankle) is sore and it is not going to go away for a week or so."
Galaxy president and general manager Alexi Lalas told ESPN television: "It is a long-term process and we want to make sure David is 100 percent.
"We have games next week, next month and over the next five years. Everyone will get their chance to see David Beckham here and across the country."